Backport PR #8303: fix bug in stale profile clean up for clusters

"clusters" tab of my notebook wasn't rendering, so checked the log, found the stack trace:

```
[...]
      File "/usr/local/lib/python3.4/site-packages/ipython-3.0.0-py3.4.egg/IPython/html/services/clusters/handlers.py", line 21, in get
...

IPython/html/services/clusters/clustermanager.py

@@ -1,162 +1,162 b''
 """Manage IPython.parallel clusters in the notebook."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 from tornado import web
 
 from IPython.config.configurable import LoggingConfigurable
 from IPython.utils.traitlets import Dict, Instance, Float
 from IPython.core.profileapp import list_profiles_in
 from IPython.core.profiledir import ProfileDir
 from IPython.utils import py3compat
 from IPython.utils.path import get_ipython_dir
 
 
 class ClusterManager(LoggingConfigurable):
 
     profiles = Dict()
 
     delay = Float(1., config=True,
         help="delay (in s) between starting the controller and the engines")
 
     loop = Instance('zmq.eventloop.ioloop.IOLoop')
     def _loop_default(self):
         from zmq.eventloop.ioloop import IOLoop
         return IOLoop.instance()
 
     def build_launchers(self, profile_dir):
         from IPython.parallel.apps.ipclusterapp import IPClusterStart
         
         class DummyIPClusterStart(IPClusterStart):
             """Dummy subclass to skip init steps that conflict with global app.
     
             Instantiating and initializing this class should result in fully configured
             launchers, but no other side effects or state.
             """
 
             def init_signal(self):
                 pass
             def reinit_logging(self):
                 pass
         
         starter = DummyIPClusterStart(log=self.log)
         starter.initialize(['--profile-dir', profile_dir])
         cl = starter.controller_launcher
         esl = starter.engine_launcher
         n = starter.n
         return cl, esl, n
 
     def get_profile_dir(self, name, path):
         p = ProfileDir.find_profile_dir_by_name(path,name=name)
         return p.location
 
     def update_profiles(self):
         """List all profiles in the ipython_dir and cwd.
         """
         
         stale = set(self.profiles)
         for path in [get_ipython_dir(), py3compat.getcwd()]:
             for profile in list_profiles_in(path):
                 if profile in stale:
                     stale.remove(profile)
                 pd = self.get_profile_dir(profile, path)
                 if profile not in self.profiles:
                     self.log.debug("Adding cluster profile '%s'", profile)
                     self.profiles[profile] = {
                         'profile': profile,
                         'profile_dir': pd,
                         'status': 'stopped'
                     }
         for profile in stale:
             # remove profiles that no longer exist
             self.log.debug("Profile '%s' no longer exists", profile)
-            self.profiles.pop(stale)
+            self.profiles.pop(profile)
 
     def list_profiles(self):
         self.update_profiles()
         # sorted list, but ensure that 'default' always comes first
         default_first = lambda name: name if name != 'default' else ''
         result = [self.profile_info(p) for p in sorted(self.profiles, key=default_first)]
         return result
 
     def check_profile(self, profile):
         if profile not in self.profiles:
             raise web.HTTPError(404, u'profile not found')
 
     def profile_info(self, profile):
         self.check_profile(profile)
         result = {}
         data = self.profiles.get(profile)
         result['profile'] = profile
         result['profile_dir'] = data['profile_dir']
         result['status'] = data['status']
         if 'n' in data:
             result['n'] = data['n']
         return result
 
     def start_cluster(self, profile, n=None):
         """Start a cluster for a given profile."""
         self.check_profile(profile)
         data = self.profiles[profile]
         if data['status'] == 'running':
             raise web.HTTPError(409, u'cluster already running')
         cl, esl, default_n = self.build_launchers(data['profile_dir'])
         n = n if n is not None else default_n
         def clean_data():
             data.pop('controller_launcher',None)
             data.pop('engine_set_launcher',None)
             data.pop('n',None)
             data['status'] = 'stopped'
         def engines_stopped(r):
             self.log.debug('Engines stopped')
             if cl.running:
                 cl.stop()
             clean_data()
         esl.on_stop(engines_stopped)
         def controller_stopped(r):
             self.log.debug('Controller stopped')
             if esl.running:
                 esl.stop()
             clean_data()
         cl.on_stop(controller_stopped)
         loop = self.loop
         
         def start():
             """start the controller, then the engines after a delay"""
             cl.start()
             loop.add_timeout(self.loop.time() + self.delay, lambda : esl.start(n))
         self.loop.add_callback(start)
 
         self.log.debug('Cluster started')
         data['controller_launcher'] = cl
         data['engine_set_launcher'] = esl
         data['n'] = n
         data['status'] = 'running'
         return self.profile_info(profile)
 
     def stop_cluster(self, profile):
         """Stop a cluster for a given profile."""
         self.check_profile(profile)
         data = self.profiles[profile]
         if data['status'] == 'stopped':
             raise web.HTTPError(409, u'cluster not running')
         data = self.profiles[profile]
         cl = data['controller_launcher']
         esl = data['engine_set_launcher']
         if cl.running:
             cl.stop()
         if esl.running:
             esl.stop()
         # Return a temp info dict, the real one is updated in the on_stop
         # logic above.
         result = {
             'profile': data['profile'],
             'profile_dir': data['profile_dir'],
             'status': 'stopped'
         }
         return result
 
     def stop_all_clusters(self):
         for p in self.profiles.keys():
             self.stop_cluster(p)