teuthology: only direct admin socket commands to active MDS
Right now our flush and scrub asok command tests are directed at mds.a, but sometimes that's not the active MDS. Find a way to always direct them at the active MDS, whichever one it is.
mds_scrub_checks: only direct ops at the active MDS
Change the config option from mds_id to mds_rank to reflect the
fact that it's the rank we want to make use of (and will continue
to want when we're doing stuff like force exporting from one rank
Signed-off-by: Greg Farnum <firstname.lastname@example.org>