mon/OSDMonitor: adjust weights up, when possible

Note when OSDs are underloaded, as well.  If that is the case, adjust the
OSD reweight value if, if possible.  (It won't always be possible since
weights are capped at 1.)

Note that we set the underload threshold to the average, as we want to
aggressively adjust weights up (back to 1.0) whenever possible.  This gets
us a more efficient mapping calculation and reduces the amount of "noise"
in the weights.

Signed-off-by: Sage Weil <sage@redhat.com>
mon/OSDMonitor: reweight-by-pg for pool(s)

Allow the reweight-by-pg to look at a specific set of pools.  If the list
is ommitted, use PGs from all pools.  This allows you to focus on a
specific pool (the one that will dominate data usage).  Otherwise things
may not be quite right because other pools may have PGs that contain
much less data.

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