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

Not returning a value is the same as returning None. In the event that
someone asks ClientManager for an attribute that doesn't exist it should
raise AttributeError in the same way as other python objects rather
than return an empty value.

Change-Id: Id0ee825e6527c831c38e3a671958ded362fb96e1

When we got picky with the auth arguments we broke using help without
any auth config supplied.  This rearranges things a bit to do the argument
checking when the deferred auth request to Identity occurs so commands
that do not need auth have a chance to live short but useful lives.

Closes-Bug: #1399588
Change-Id: I8ceac491cf65e25eddb62ab2713f471fe686756d

Change-Id: Ieff86f841623e1ce34b79c66bc5e8b1b239fa3e9

Include direction users pass in the error msg to help users know
which part of the argument is troublesome.

Change-Id: I796a85fbf40f6ddf544fb52a61f967e1914abdcc
Closes-Bug: #1420732