task: add mds_client_recovery

This task exercises the CephFS session recovery
behavior.

Signed-off-by: John Spray <john.spray@redhat.com>
task: fix admin_socket for non-int ids

This was working for OSDs and for my config
where MDS happened to have numeric ID, but
in general service IDs are strings, not ints.

Signed-off-by: John Spray <john.spray@redhat.com>