Temporarily add eventlet to requirements

We absolutely do not want this to be a requirement, but for the moment
the rabbit and qpid drivers import eventlet and avoid using it at
runtime. This should be cleaned up shortly and we can remove the dep
again.

Change-Id: Iaef75964a502598a94d95aa77f0e44d1f7faba50
Merge "Temporarily add eventlet to requirements"