Make the Identity Manager plugable where appropriate

For deployments with LDAP as the backend for users, or other federated sources, we can't get all the data we need from Keystone, nor can we edit/manage users. For those deployments the identity manager needs to be plugable so they can use one that can talk to their user store (LDAP, AD, etc).

This way Adjutant can still manage users in OpenStack land, but handle users as expected behind the scenes in the right places.

The IdentityManager has always been an abstraction for this purpose, but was never made pluggable since the usecase wasn't needed. We should just do it and have it ready even if we only have the default keystone manager for now.