Rename requirements files to standard names.

Change-Id: I0385ac9f8336a8c2e45b0fa6ed062b6b23a182aa
Reviewed-on: https://review.openstack.org/33351
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins