Support building multiple pypi mirrors.

Use atomic renames when creating the mirror to avoid races.

Change-Id: Id3a75832d2a71a71253470ffae7499a3946e8ee6
Reviewed-on: https://review.openstack.org/23771
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins