mgr/cephadm: wait for active MDS during fail_fs upgrade

When fail_fs is enabled with max_mds > 1, _prepare_for_mds_upgrade()
issued fs fail and then continued unconditionally, skipping the
up:active wait. _complete_mds_upgrade() also set joinable=true without
waiting for in-rank MDS to recover.

Wait for all in-rank MDS to reach up:active before proceeding with the
upgrade and after re-joining the filesystem. Add fs.ready to the
mds_upgrade_sequence verify task so ceph-fuse teardown does not race
with MDS still in up:rejoin.

Fixes: https://tracker.ceph.com/issues/77835

Signed-off-by: Kobi Ginon <kginon@redhat.com>