mon: Sanely set the default CRUSH rule when creating pools in stretch mode

mon: Sanely set the default CRUSH rule when creating pools in stretch mode

If we get a pool create request while in stretch mode that does not explicitly
specify a crush rule, look at the stretch-mode pools and their rules, and
select the most common one.

Also update set_up_stretch_mode.sh to add a few more rules that let me test
this locally.

Fixes: https://tracker.ceph.com/issues/51270
Signed-off-by: Greg Farnum <gfarnum@redhat.com>