qa: leave one standby available to avoid warning
Four file systems will use all MDS and generate this warning:
2020-11-02T03:48:33.407 INFO:teuthology.orchestra.run.smithi003.stdout:2020-11-02T03:24:21.817337+0000 mon.a (mon.0) 481 : cluster [WRN] Health check failed: insufficient standby MDS daemons available (MDS_INSUFFICIENT_STANDBY).
Fixes: https://tracker.ceph.com/issues/23718
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit
59451923d31f5e4f707aa6d22ececc8edd395ca9)