OSDMonitor: skip new pools in update_pools_status() and get_pools_health()

New pools won't be full.  mon->pgmon()->pg_map.pg_pool_sum[poolid] will
implicitly create an entry for poolid causing register_new_pgs() to assume that
the newly created pgs in the new pool are in fact a result of a split
preventing MOSDPGCreate messages from being sent out.

Fixes: #4813
Backport: cuttlefish
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>