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>
the idea is that the osdmap should never be unable to tell us the name of a pool that currently exists (assuming 'p' in this context is from the same instance of the osd map as 'osdmap') -- in that sense it really is an assertion.  However I didn't comprehensively check this: is it actually possible for this not to be the case?