Use Instance Objects for Start/Stop

This patch makes the start and stop operations use the Instance
object instead of passing SQLA-derived dicts over RPC. Until
something more sophisticated is needed (and developed), it also
adds a nova.object.register_all() function which just triggers
imports of all the object models so that they are registered.
When adding a new object type, that register function should be
updated appropriately.

Related to bp/unified-object-model

Change-Id: I3c8d9cba07d34097a279502062906de802d19d1f
Merge "Use Instance Objects for Start/Stop"