doc: only OpenStack with OVH is supported
doc: only OpenStack with OVH is supported

The other providers are likely to work but since they are not actively
tested, it is better no to claim they do.

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

ceph/teuthology,SUSE/teuthology,dmick/teuthology,SUSE/teuthology,ceph/teuthology,SUSE/teuthology,dmick/teuthology,dmick/teuthology,ktdreyer/teuthology,ktdreyer/teuthology