Move OSC auth plugins so they can be found

The plugin detection at the top of openstackclient.api.auth did not
detect the plugins at the bottom of that file because, surprise,
they had not been declared yet so the entry points were ignored.

Move both plugin subclasses into openstackclient.api.auth_plugin.

Fix a problem with the password callback that was otherwise made
worse with this change.

Closes-Bug: 1428912
Change-Id: Idc3b72534071e0013c8922884a8bc14137509a0f

openstackclient/shell.py

@@ -188,12 +188,6 @@ class OpenStackShell(app.App):
             description,
             version)
 
-        # service token auth argument
-        parser.add_argument(
-            '--os-url',
-            metavar='<url>',
-            default=utils.env('OS_URL'),
-            help='Defaults to env[OS_URL]')
         # Global arguments
         parser.add_argument(
             '--os-region-name',