Add ability to specify the env dict for launching the kernel

This adds ability to use different dict other than os.environ
as the environment for launching the kernel

Author: Pankaj Pandey

jupyter_client/manager.py

@@ -228,7 +228,7 @@ def start_kernel(self, **kw):
         # build the Popen cmd
         extra_arguments = kw.pop('extra_arguments', [])
         kernel_cmd = self.format_kernel_cmd(extra_arguments=extra_arguments)
-        env = os.environ.copy()
+        env = kw.pop('env', os.environ).copy()
         # Don't allow PYTHONEXECUTABLE to be passed to kernel process.
         # If set, it can bork all the things.
         env.pop('PYTHONEXECUTABLE', None)