Avoid creating default connection dir if not needed

If a connection file is specified with an absolute path via the '-f'
command line option, ipykernel still makes sure that the default connection
directory exists, even though it is not needed. Instead, simply create the
directory for the path to the connection file that we end up actually
using.

This introduces a slight change in behaviour in that if one specifies a
connection file via the '-f' option and the given directory does not
exist, it will be created instead of an error being generated.

Author: Armin Burgmeier

ipykernel/kernelapp.py

@@ -115,9 +115,7 @@ class IPKernelApp(BaseIPythonApplication, InteractiveShellApp,
     # connection info:
     connection_dir = Unicode()
     def _connection_dir_default(self):
-        d = jupyter_runtime_dir()
-        ensure_dir_exists(d, 0o700)
-        return d
+        return jupyter_runtime_dir()
 
     @property
     def abs_connection_file(self):
@@ -201,7 +199,9 @@ def init_connection_file(self):
             self.connection_file = filefind(self.connection_file, ['.', self.connection_dir])
         except IOError:
             self.log.debug("Connection file not found: %s", self.connection_file)
-            # This means I own it, so I will clean it up:
+            # This means I own it, and I'll create it in this directory:
+            ensure_dir_exists(os.path.dirname(self.abs_connection_file), 0o700)
+            # Also, I will clean it up:
             atexit.register(self.cleanup_connection_file)
             return
         try: