Bring in a newer optional eventlet

Since the openstack requirements repo just accepted eventlet>=0.15.1
we might as well also use that version in our optional requirements
and remove a piece of code that was dealing with a bug that was fixed
in eventlet 0.15.

Change-Id: I9b4f9061c7adb7d72315315f41bb0d742b6f56b5
Merge "Bring in a newer optional eventlet"