Deprecate direct access to clients methods

This change deprecates access to methods like clients.nova()
replacing them with calls to clients.client('nova')

This allows the memoization to be done in one place (the client()
method) and also establishes the preferred API for accessing clients
that are contributed by plugins.

All calls should now be going through the new client() method.
Any calls to the old methods will result in a deprecated log
warning.

Change-Id: I3dc45aaed2655536ed1558e03f468b0733527815
Merge "Deprecate direct access to clients methods"