Allow configuring compute tenant isolation

This configuration setting may be required for running tempest against
an existing openstack cloud.

Conflicts:
	spec/classes/tempest_spec.rb

Change-Id: I852ae3d3ab54714205c9a78fcee1506654b66efe
(cherry picked from commit 0bdf6b88e157d86012f75059bd1f6fabf8a36a73)