Add a requirements testing script.

Can be used to verify that updated requirements match those
specified in openstack/requirements.

Change-Id: Ieec680f0373786b75eaf947af15c60a78c763180
Reviewed-on: https://review.openstack.org/23843
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins