mon/PGMonitor: set floor below which we do not warn about objects/pg

If a cluster has very few objects, do not generate warnings when the 
objects/pg for a pool diverges from the cluster average.  This avoids 
spurious errors when you have a relatively empty cluster and a lone pool with
a modest number of objects is too far off the (mostly meaningless) 
cluster-wide average.

Fixes: #6521 Signed-off-by: Sage Weil <sage@inktank.com>