mds: add balance_automate fs setting

mds: add balance_automate fs setting

To turn off the automatic ("default") balancer in multiple MDS clusters. The
new default is "off" as the balancer  is a constant source of problems and
surprise for administrators trying multiple actives. Instead, it should be a
deliberate decision to turn it on and usually with customization like the
"bal_rank_mask" setting or pinning.

Fixes: https://tracker.ceph.com/issues/61378
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>