Add third-party library for py-rrdtool, used by kwapi.

Change-Id: I57d789add8e0cdb4ae789b803750bc30eb0f480c
Reviewed-on: https://review.openstack.org/19716
Reviewed-by: François Rossigneux <francois.rossigneux@inria.fr>
Reviewed-by: Julien Danjou <julien@danjou.info>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins