crushtool: add --set-subtree-class; do not set class via --reclassify-root

crushtool: add --set-subtree-class; do not set class via --reclassify-root

Sometimes we don't want the --reclassify-root to set the class of every
device because a small number of them are (correctly) a different class.
Allow both behaviors by adding a new, separate command to set the class
of all devices beneath a point in the hierarchy and do not implicitly do
that relabeling as part of --reclassify-root.

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