Handle setup_requires in mirror script.

Change-Id: Ice9ccad1dc28ecd5a44430839a374632f9912514
Reviewed-on: https://review.openstack.org/24862
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins