Engine as a standalone process

Refactored engine to run as a standalone process. The engine is
modified to use the RPC client/server design pattern from
oslo.messaging. A new launch option is included to run the
engine separately. The API server communicates with the engine
via the RPC client. A pecan hook is used to pass a common client
instance with appropriate transport configuration to the API.

Change-Id: I9ca757aedad764fd38645a5b858800d69808bb95
Implements: blueprint mistral-engine-standalone-process