Gate requirements on ability to install deps.

Gate openstack/requirements on its ability to install the requirements
it specifies. This will help ensure that the mirror scripts can use this
repository to seed the mirror(s).

Change-Id: I5647a0ca32fd141c94a7e6afce1759fb1ad34442
Reviewed-on: https://review.openstack.org/23751
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins