add python-mock to testonly package list

Python-mock is needed by most of OpenStack for testability. Add
this is a testonly package. This should fix current issues with
functional tests in OpenStack that use devstack, but not tox to
run their tests.

Closes-Bug: #1341507

Change-Id: I26cbe42ace75554bae993a6d849c3b4ac649c78e