mon: test the crush ruleset when creating a pool

mon: test the crush ruleset when creating a pool

We want to fix the following scenario:

 * an erasure code plugin (or another part of the code) creates a
   ruleset
 * the ruleset crashes during mapping (for whatever reason)
 * ceph osd pool create uses the bugous ruleset
 * the monitors try to do mapping a crash

Having a bugous ruleset in the crush map is very difficult prevent. The
catastrophic event of using it with a newly created pool can however be
prevented by calling the CrushTester just before creating the pool and
after all implicit or explicit crush ruleset creation happened.

http://tracker.ceph.com/issues/11814 Fixes: #11814

Signed-off-by: Loic Dachary <ldachary@redhat.com>