Use requirements-core for os/requirements.

Change-Id: I767e41ac4964bf8dff4b92c9cc9082a67f03a44a
Reviewed-on: https://review.openstack.org/27577
Reviewed-by: Mark McLoughlin <markmc@redhat.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins