Update placement groups when adding new ceph nodes

When deploying MOS with Ceph cluster, pg_num and pgs_num parameters are only calculated once (when creating the pools). We need to recalculate it every time we add new osd nodes, as well as we have to execute "ceph osd pool set {pool-name} pg_num {pg_num}" and "ceph osd pool set {pool-name} pgp_num {pgp_num}" for all existing pools if necessary. It would be beneficial to delay the rebalancing until all new nodes are added and the pg_num/pgp_num values have been updated.