Fixing unticketed bug in ipcluster.py.

Previously on Windows we were looking for the Python executable that
was first on the path.  This was causing problems when the python
running ipcluster was a different one.  I am now finding the proper
python exe by looking at sys.executable.

IPython/kernel/scripts/ipcluster.py

@@ -1,4 +1,4 b''
-#!/usr/bin/env python
+ #!/usr/bin/env python
 # encoding: utf-8
 
 """Start an IPython cluster = (controller + engines)."""
@@ -184,8 +184,10 b' class ControllerLauncher(ProcessLauncher):'
             from IPython.kernel.scripts import ipcontroller
             script_location = ipcontroller.__file__.replace('.pyc', '.py')
             # The -u option here turns on unbuffered output, which is required
-            # on Win32 to prevent wierd conflict and problems with Twisted
-            args = [find_exe('python'), '-u', script_location]
+            # on Win32 to prevent wierd conflict and problems with Twisted.
+            # Also, use sys.executable to make sure we are picking up the 
+            # right python exe.
+            args = [sys.executable, '-u', script_location]
         else:
             args = ['ipcontroller']
         self.extra_args = extra_args
@@ -204,8 +206,10 b' class EngineLauncher(ProcessLauncher):'
             from IPython.kernel.scripts import ipengine
             script_location = ipengine.__file__.replace('.pyc', '.py')
             # The -u option here turns on unbuffered output, which is required
-            # on Win32 to prevent wierd conflict and problems with Twisted
-            args = [find_exe('python'), '-u', script_location]
+            # on Win32 to prevent wierd conflict and problems with Twisted.
+            # Also, use sys.executable to make sure we are picking up the 
+            # right python exe.
+            args = [sys.executable, '-u', script_location]
         else:
             args = ['ipengine']
         self.extra_args = extra_args