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 other than
   'master' did not work.

Change-Id: I723536d64277d8ad2f82aa7503523347c573b3f2