Remove local engine

The local engine is replaced in process executor using the oslo.messaging
fake transport.  The local engine module is removed here along with related
unit tests. Some unit tests are rewritten to use the ScalableEngine. An
EngineTestCase is included in the base module of tests to simplify engine
testing.

Change-Id: I1a49a53eac87a209660c493fd7c3fe3e914fd092
Implements: blueprint mistral-inproc-executor