Message-oriented worker-based flow with kombu

* Implemented Worker to be started on remote host for
  handling tasks request.
* Implemented WorkerTaskExecutor that proxies tasks
  requests to remote workers.
* Implemented Proxy that is used for consuming and
  publishing messages by Worker and Executor.
* Added worker-based engine and worker task executor.
* Added kombu dependency to requirements.
* Added worker-based flow example.
* Added unit-tests for worker-based flow components.

Implements: blueprint worker-based-engine
Change-Id: I8c6859ba4a1a56c2592e3d67cdfb8968b13ee99c
Merge "Message-oriented worker-based flow with kombu"