account the number of osds when setting pg_num/pgp_num

[Ricardo Rocha]

If not given explicitly as a param, we currently set the pg_num to a default (64) which does not take into account the number of OSDs in the cluster.

As described in:
http://ceph.com/docs/master/rados/operations/placement-groups/

there's a recommendation of setting this value to (OSDs * 100) / replicas.

We should look at a way to put this recommendation in practice, which requires keeping track of the number of available OSDs in puppet (not there now).