Individual plugin CLI registering

Split the functions that load the auth plugins from CLI so that they can
be used on a specific plugin. The intention here is to be able to turn
the existing authentication options in shells into a new auth plugin and
have that be loadable rather than maintain separate paths through the
shells.

Change-Id: I3dd5a8ed183d843246b1add3dfbf591ba4e2f94c
Merge "Individual plugin CLI registering"