mgr: add simple controller to adjust pg[p]_num_actual

mgr: add simple controller to adjust pg[p]_num_actual

This is a pretty trivial controller.  It adds some constraints that were
obviously not there before when the user could set these values to anything
they wanted, but does not implement all of the "nice" stepping that we'll
eventually want.  That can come later.

Splits:
- throttle pg_num increases, currently using the same config option
(mon_osd_max_creating_pgs) that we used to throttle pg creation
- do not increase pg_num until the initial pg creation has completed.

Merges:
- wait until the source and target pgs for merge are active and clean
before doing a merge.

Adjust pgp_num all at once for now.

Signed-off-by: Sage Weil <sage@redhat.com>