Add support for requirements.txt.

Change-Id: I80874548a5e1391055150752f18ee9f26c8012ed
Reviewed-on: https://review.openstack.org/18891
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins