Rename po-file-update.sh to pot-file-update.sh
update-pot.sh: Support branch option
import-trans: Only fetch languages over 20% completion
import-trans-for-propose: Prepare a translation import patch with threshold

import-trans-for-propose.sh prepares a Horizon patch to import
translation. It only imports languages whose completeness is
over the specified threshold (95%).

Transifex client has an option "--minimum-perc" but it only checks
a single transifex resource. Horizon has three translation resources
(horizon, horizon-javascript, openstack_dashboard) and I need to
take into account completion percentage of all thress resources.
completion_check.py does this job.