use App.instance() in launch_new_instance (parallel apps)

This way later calls to App.instance() will return the running
application, whereas calling App() would create an isolated
Application object.

IPython/parallel/apps/ipclusterapp.py

@@ -511,7 +511,7 b' class IPBaseParallelApplication(Application):'
 
 def launch_new_instance():
     """Create and run the IPython cluster."""
-    app = IPBaseParallelApplication()
+    app = IPBaseParallelApplication.instance()
     app.initialize()
     app.start()
 

IPython/parallel/apps/ipcontrollerapp.py

@@ -389,7 +389,7 b' class IPControllerApp(BaseParallelApplication):'
 
 def launch_new_instance():
     """Create and run the IPython controller"""
-    app = IPControllerApp()
+    app = IPControllerApp.instance()
     app.initialize()
     app.start()
 

IPython/parallel/apps/ipengineapp.py

@@ -260,7 +260,7 b' class IPEngineApp(BaseParallelApplication):'
 
 def launch_new_instance():
     """Create and run the IPython engine"""
-    app = IPEngineApp()
+    app = IPEngineApp.instance()
     app.initialize()
     app.start()
 

IPython/parallel/apps/iploggerapp.py

@@ -86,7 +86,7 b' class IPLoggerApp(BaseParallelApplication):'
 
 def launch_new_instance():
     """Create and run the IPython LogWatcher"""
-    app = IPLoggerApp()
+    app = IPLoggerApp.instance()
     app.initialize()
     app.start()