Paralleize quantum unit testing:

This change switches run_tests.sh to use testr
in parallel mode, instead of nosetests, in order
to enable unit tests to run in parallel.

A similar change was recently merged to nova:
  https://review.openstack.org/#/c/15078/

Some tests required updating to remove inter-testcase
dependencies.

bug #1099694

Change-Id: Idfb923d424342a07dcba88d70b971683f549f763