mon/OSDMonitor: allow pg_num to adjusted up or down via pg[p]_num_target

mon/OSDMonitor: allow pg_num to adjusted up or down via pg[p]_num_target

The CLI now sets the *_target values, imposing only the subset of constraints that
the user needs to be concerned with.

new "pg_num_actual" and "pgp_num_actual" properties/commands are added that allow
the underlying raw values to be adjusted.  For the merge case, this sets
pg_num_pending instead of pg_num so that the OSDs can go through the
merge prep process.

A controller (in a future commit) will make pg[p]_num converge to pg[p]_num_target.

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