Refactor engine to use plugins

Refactor current engine process to instantiate the concrete engine using
stevedore. The current engine uses importutils to load the concrete engine.
Since the executor is tightly coupled to the engine, it is also refactored
to use stevedore. The ScalableEngine is renamed to DefaultEngine and moved
to the mistral.engine.drivers.default namespace. The setup.cfg file is
updated to include entry points for the engine and executor plugins.

Change-Id: Ia9d15b25ca96387e5ac22c1a86cffc7a816e92fd
Implements: blueprint mistral-engine-plugin
Merge "Refactor engine to use plugins"