Make heat.engine.clients a package

As blueprint client-plugins progresses, the code in __init__.py
will move into plugin modules, until the only package initialising
code remaining will be the following:
- plugin initialise
- Clients class which memoizes client plugins for a given context
- deprecated clients methods (until deleted later)

Partial-Blueprint: client-plugins
Change-Id: Ic8f67e3519b10c0d3d204e15e5ad72376c50a1dd
Merge "Make heat.engine.clients a package"