Remove _factory methods from auth plugins

This was a simple factory that would give compatibility for the existing
client to load up the appropriate auth plugin. A more robust plugin
loading mechanism is coming for this and having it available encourages
other auth plugins that they should be using that where they shouldn't.

Just remove it from the auth plugin class. It shouldn't be used by
anyone else so lets keep it on the client objects.

Blueprint: plugin-params
Change-Id: I0618b646f302300d41c7dd7153a1c0bdc237a745
Merge "Remove _factory methods from auth plugins"