Support TEMPEST_BRANCH with tag name

TEMPEST_BRANCH which is mostly set as master so
that Tempest master is run to test the env. With
stable branch going to EM state and Tempest master
might not work due to incompatibility of code or
requirements. In that case we pin the Tempest so that
older Tempest can be used for their testing.

Till now for ocata, pike and, queens we used the gerrit style
ref to pin the Tempest which is not preferred way. We should be
able to use the tag name on TEMPEST_BRANCH.

This commit explicitly checkout the tag set in TEMPEST_BRANCH
as git_clone does not checkout the tag directly until RECLONE
is true or tempest dir does not exist.

After this stable branch or job can set the tag directly with name.
For exmaple: TEMPEST_BRANCH=23.0.0.

Change-Id: Ic777e4b56c4932dde135ac909cb5c6f4a7d5cc78

lib/tempest

@@ -686,6 +686,11 @@ function install_tempest {
     git_clone $TEMPEST_REPO $TEMPEST_DIR $TEMPEST_BRANCH
     pip_install 'tox!=2.8.0'
     pushd $TEMPEST_DIR
+    # NOTE(gmann): checkout the TEMPEST_BRANCH in case TEMPEST_BRANCH
+    # is tag name not master. git_clone would not checkout tag because
+    # TEMPEST_DIR already exist until RECLONE is true.
+    git checkout $TEMPEST_BRANCH
+
     tox -r --notest -efull
     # NOTE(mtreinish) Respect constraints in the tempest full venv, things that
     # are using a tox job other than full will not be respecting constraints but