Fix installing tempest plugins
Fix installing tempest plugins

When running stack.sh locally on stable branches
with tempest enabled and TEMPEST_PLUGINS set,
devstack will try to fetch master branch of requirements
and that fails if branch is not tracked.

Change-Id: Ia1ae6869a8fede2af5cd7c875e0946b6a75eb518
Closes-Bug: #1820051

openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack,dtroyer/devstack,nawawi/openstack,dtroyer/devstack