mon/OSDMonitor: add 'osd repeer <pgid>' command

mon/OSDMonitor: add 'osd repeer <pgid>' command

Selecting force peering on a single PG.  In reality this probably induces
*2* interval changes.

Note that in the case of a single OSD cluster we can't actually force a
repeer on a single PG because the pg_temp code is pretty robust about
filtering out redundant or meaningless changes, so we can't pg_temp our
way into a new interval if there are no other OSDs to switch to and the
code also prevents an empty pg_temp.

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