Add a 'greenio' oslo.messaging executor

Add a new oslo.messaging executor which adds a new capability to the
eventlet executor - the ability to dispatch requests to asyncio
coroutines running on greenio's greenlet based asyncio event loop.

This will allow OpenStack services to begin a gradual conversion to
asyncio while still using eventlet as its event loop.

Change-Id: Iffba8e3e41e2e4b9b97a88b3ec856e738b52eadf
blueprint: greenio-executor
Merge "Add a 'greenio' oslo.messaging executor"