Easy testing with alternate keystoneclient

It wasn't easy to test with an alternative keystoneclient, just
the checked out ones.

This change makes it easy to test with an alternative local
keystoneclient, just set KSCTEST_PATH to the path:

 KSCTEST_PATH=/opt/stack/python-keystoneclient tox -e py27 KcOptTestCase

Change-Id: I0af834d2861220b72b93265a1ddbfd8bc3d13d49
Merge "Easy testing with alternate keystoneclient"