openstack: use --limit 2000 with openstack image list command
openstack: use --limit 2000 with openstack image list command

The --limit defaults to 25, which is way too low.

Fixes: https://github.com/SUSE/teuthology/issues/82
Signed-off-by: Loic Dachary <b6eae37fc74638590e3f9030895d42446505d41b@dachary.org>
Signed-off-by: Nathan Cutler <f9d8c66a4e898e67db786769b5b347b640e1a9a6@suse.com>

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