task/ceph: move set_max_mds into cephfs_setup

Move the calculation of active MDS count out
of the generic service setup and into the
filesystem setup.

Fixes: #8750

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #285 from ceph/wip-8750-jcsp

task/ceph: move set_max_mds into cephfs_setup

Reviewed-by: Sage Weil <sage@redhat.com>