Always used the library packaged mock
Always used the library packaged mock

The library packaged mock seems to work
better across python versions, so instead of
trying to mix the system one and the library
one always prefer the library one (which should
work in all supported python versions).

Change-Id: Id6ada69df9961a6b7511316e2f75ef9ee5f0b72b

jimbobhickville/taskflow,openstack/taskflow,openstack/taskflow,jimbobhickville/taskflow