mon: name instead of id in "has tiers" message

Instead of "Pool foo has tiers 1 2" print
"Pool foo has tiers bar baz".

Signed-off-by: John Spray <jspray@redhat.com>
Merge pull request #1997 from ceph/wip-tiermsg

mon: name instead of id in "has tiers" message

Reviewed-by: Loic Dachary <loic@dachary.org>