Only skip openstack tarballs in requirements check

We really only want to allow openstack tarball requirements to be added
without checking, not just any tarball.

Change-Id: I0ce458c95dc2fba07100b921d094ff174ed7da57
Reviewed-on: https://review.openstack.org/36070
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins