Cleanup repo revision handling.

 * The use of the parameter 'version_to_test' is ambiguous, and
   implies that it is possible to specify an openstack release name
   like 'folsom' or 'grizzly'.  In fact, what is being specified
   is a git revision, branch, or tag.  The upstream branches are
   named 'stable/folsom' or 'stable/grizzly', and changing the name
   to 'tempest_repo_revision' should make it easier to deduce that
   such values are required.
 * Do not provide a default revision of 'master' - use the repo
   default instead.
 * Remove source tree cleanup for the 'folsom' branch.  There is
   no such branch upstream, it is 'stable/folsom', and besides,
   specifying the branch will provide a tempest tree compatible
   with the folsom release.

Change-Id: Iab117cffea22b50480bd88e3ea4d5eba55a69fe5