Implement keystone client plugin

This moves the client creation code out of Clients._keystone() into
its own client plugin.
This is the last client to convert to a plugin, so supporting code
to fetch a client from a non-plugin has been removed.
In a later change it would be worth considering moving methods
in heat_keystoneclient.KeystoneClient into KeystoneClientPlugin
so that calling Clients.client('keystone') returns the actual keystone
client rather than a wrapper.

The barbican TestClient has been removed; it had limited practical
value since it mocked all of the internal calls of the method it was
testing.

Change-Id: Ie4a616d6a2b056fe59877e2bec325f02b6d7a693
Merge "Implement keystone client plugin"