Implement marconi client plugin

This moves the client creation code out of a Clients subclass into
its own client plugin.

The base package has been renamed to heat_marconi since stevedore
is used for client plugin loading and the base package will be
installed by setuptools, the package marconi might clash with
the actual marconi project.

Change-Id: Ie824d961fcfa9dafd72cc40ccf207d1c1e0501ad