Fix warning in run_tests.sh --makemessages

Specify --extension=html,txt,csv only for openstack_dashboard.
For djangojs, --extension should be 'js'.
This is the cause of the warning.

Change-Id: Ia5ac03fa82c18356b8d210005f8696ae148981bd
Closes-Bug: #1243526
Merge "Fix warning in run_tests.sh --makemessages"