Add openstack-requirements to #openstack-infra.

Change-Id: Ia15ddcdd587043b2d10524a839bad793e52d58c3
Reviewed-on: https://review.openstack.org/24801
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins