Test only projects listed in projects.txt.

Trove stable/havana is not working and blocking
python-neutronclient>=2.3.4 update in stable/havana requirements.
Related changes:
 Ia8eafb4a5c8b950ae807464b166d44b435b824ad
 If0eab1b5dd102cc5ceb7720a8465ee47a708839d
 I92bea7c0868b9e8fc021975a5f3f3458a70d25e6

Change-Id: I77126fcfc95bab78f17f5818843517968d4f5ba0
Related-Bug: #1280941
Merge "Test only projects listed in projects.txt."