Make Identity client load like the others

This does a couple of things:
* Loads the Identity client module in the same manner as the other
  'base' clients (where 'base' == 'included in the OSC repo')
* Changes the entry point group name for the base clients to
  'openstack.cli.base'.  The extension group name remains the same.
* Loads the base modules first followed by the extension modules.
  This load order ensures that the extension module commands are all
  loaded _after_ the base commands, allowing extensions to now override
  the base commands.

Change-Id: I4b9ca7f1df6eb8bbe8e3f663f3065c2ed80ce20b
Merge "Make Identity client load like the others"