mon/PGMonitor: add 'pg dump pgs_brief' subcommand

It is useful to map OSDs to PGs and vice-versa; pg dump gives that
information, but gives a lot of other stuff.  This is the same dump
as pg dump pgs, but omitting everything except pgid, state, and
osd up and acting sets.

Signed-off-by: Dan Mick <dan.mick@inktank.com>