mon: add 'osd crush {get,set}-tunable <name> [value]' commands

For now, just add the straw_calc_version tunable.

Signed-off-by: Sage Weil <sage@redhat.com>
crush/builder: a note about the original crush_calc_straw()

Signed-off-by: Sage Weil <sage@redhat.com>
crush: set straw_calc_version=1 for default+optimal; do not touch for presets

When using the presets for compatibility (i.e., based on version), do not
touch the straw behavior, as it does not affect mapping or compatibility.
However, make a point of setting it by default and for optimal.

For most users, this means that they will not see any change unless they
explicitly enable the new behavior, or switch to default or optimal
tunables.  The idea is that if they touched it, they shouldn't be
too surprised by the subsequent data movement.

Signed-off-by: Sage Weil <sage@redhat.com>
mon: 'osd crush reweight-all'

This corresponds to the crushtool --reweight command.

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