Add greenio dependency

The greenio project will be used in OpenStack to support executing
trollius coroutines in greenlet. The first user will be the Oslo
Messaging project: see the "greenio executor" spec (accepted for Juno).
Later, at least Ceilometer will be modified to use explicit trollius
coroutines instead of implicit eventlet greenthreads.

greenio 0.6 is the first version supporting trollius, and it requires at
least trollius 1.0.

For your information, I am the maintainer of the trollius project and I
am working with Yury Selivanov on greenio. I am also working directly on
the tulip project and asyncio in Python 3.4 and 3.5.

blueprint greenio-executor

Change-Id: I564e6eb6842cfb0d7f4c29a319071667a695e99a
Merge "Add greenio dependency"