Refactor the local engine to use an in process executor

Refactored launch script to start the API and executor on the same process
so the transport for a oslo.messaging fake driver can be shared. A transport
attribute is added to the abstract engine as a class attribute to reference
the transport object.

Change-Id: I4b8684ebded9eb993749f2503380e078087edd7d
Implements: blueprint mistral-inproc-executor
Merge "Refactor the local engine to use an in process executor"