common/options: update mon_crush_min_required_version=hammer

common/options: update mon_crush_min_required_version=hammer

Hammer is a good point to set the minimum at:

 - it includes straw2 and vary_r, which are the worst offenders among the
   old tunables
 - it does not include chooseleaf_stable, which improves placement
   considerably but *also* shuffles basically all data in the cluster.

Getting everyone up to straw2 support means we can also proscribe the
'ceph osd crush set-all-straw-buckets-to-straw2' command.

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