mon/OSDMonitor: reweight-by-pg

This is just like reweight-by-utilization, but looks purely at the PG to
OSD mapping, not at the number of bytes used on the target disks.  This
allows the reweighting to be done before any data is written into the
cluster, when no data will need to migrate as a result of the reweight.

Signed-off-by: Sage Weil <sage@redhat.com>
qa/workunits/cephtool/test.sh: test reweight-by-pg

Signed-off-by: Sage Weil <sage@redhat.com>
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>
mon/OSDMonitor: respect CRUSH weights for reweight-by-pg

Do not assume that all OSDs are weighted equally for reweight-by-pg.

Note that reweight-by-utilization already reweights based on the size of
the OSD volume; we presume that this is already reflected by the CRUSH
weights.

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