Ensure FS (mds) availability during maintenance

Currently, microceph assumes the cluster only need at least one MDS service for maintenance mode. That's probably insufficient for production cluster because there could be more than one FS volumes are being consumed (one mds per fs volume). 1 MDS service may not be able to sustain FS IO requests. Microceph maintenance mode should have a better check for MDS services.

_Originally posted by @UtkarshBhatthere in https://github.com/canonical/microceph/pull/565#discussion_r2227536120_