Implement single (non-scalable) engine

 * New engine for running workflow (w/o rabbit)
 * Engine constructs a workflow
 * REST API action
 * AMQP action (not finished)
 * Task and execution state update
 * Added unit tests for this engine

Partially implements blueprint: mistral-poc
Partially implements blueprint: mistral-workflow-execution

Change-Id: I0e2b1a25f4281f77dad07a6f5241025be0963ed7
Merge "Implement single (non-scalable) engine"