allow launcher specification by prefix alone

e.g. ipcluster start --engines=SGE

IPython/parallel/apps/ipclusterapp.py

@@ -275,19 +275,22 b' class IPClusterEngines(BaseParallelApplication):'
         self.init_launchers()
     
     def init_launchers(self):
-        self.engine_launcher = self.build_launcher(self.engine_launcher_class)
+        self.engine_launcher = self.build_launcher(self.engine_launcher_class, 'EngineSet')
         self.engine_launcher.on_stop(lambda r: self.loop.stop())
     
     def init_signal(self):
         # Setup signals
         signal.signal(signal.SIGINT, self.sigint_handler)
     
-    def build_launcher(self, clsname):
+    def build_launcher(self, clsname, kind=None):
         """import and instantiate a Launcher based on importstring"""
         if '.' not in clsname:
             # not a module, presume it's the raw name in apps.launcher
+            if kind and kind not in clsname:
+                # doesn't match necessary full class name, assume it's
+                # just 'PBS' or 'MPIExec' prefix:
+                clsname = clsname + kind + 'Launcher'
             clsname = 'IPython.parallel.apps.launcher.'+clsname
-        # print repr(clsname)
         try:
             klass = import_item(clsname)
         except (ImportError, KeyError):
@@ -422,8 +425,8 b' class IPClusterStart(IPClusterEngines):'
     aliases = Dict(start_aliases)
 
     def init_launchers(self):
-        self.controller_launcher = self.build_launcher(self.controller_launcher_class)
-        self.engine_launcher = self.build_launcher(self.engine_launcher_class)
+        self.controller_launcher = self.build_launcher(self.controller_launcher_class, 'Controller')
+        self.engine_launcher = self.build_launcher(self.engine_launcher_class, 'EngineSet')
         self.controller_launcher.on_stop(self.stop_launchers)
     
     def start_controller(self):