Allow tempest repo uri and revision configuration

 * Different revisions of tempest are required for different
   releases of openstack, so it is helpful to be able to choose
   the git repo and repo revision that will be deployed.
 * The vcsrepo submodule is updated so that remote revisions are
   supported - previously providing a git revision did not work.

Change-Id: If89c06d7c909dd4e5cc7b5b07291bf526a22ea70
Merge "Allow tempest repo uri and revision configuration"