openstack: throttling helps the instance running the cluster
openstack: throttling helps the instance running the cluster

The instance throttling (not launching more than X instances per minute)
helps the instance running the teuthology cluster when running multiple
workers. The workload does not spike when launching a suite and that
allows to run more workers on a machine with the same hardware configuration.

Signed-off-by: Loic Dachary <b6eae37fc74638590e3f9030895d42446505d41b@dachary.org>

ceph/teuthology,dmick/teuthology,dmick/teuthology,caibo2014/teuthology,ktdreyer/teuthology,dreamhost/teuthology,dmick/teuthology,dreamhost/teuthology,SUSE/teuthology,ceph/teuthology,SUSE/teuthology,caibo2014/teuthology,SUSE/teuthology,robbat2/teuthology,robbat2/teuthology,ktdreyer/teuthology