mgr/pg_autoscaler: default to pg_num[_min] = 16

mgr/pg_autoscaler: default to pg_num[_min] = 16

4 or 8 PGs doesn't provide much parallelism at baseline.  Start with 16
and set the floor there; that's a more reasonable number of OSDs that
will be put to work on a single pool.

Note that there is no magic number here.  At some point someone has to
tell Ceph if an empty pool should get lots of PGs across lots of devices
to get the full throughput of the cluster.  But this will be a bit less
painful/surprising for users.

Fixes: https://tracker.ceph.com/issues/42509
Signed-off-by: Sage Weil <sage@redhat.com>