mon: check cluster features before rule create-erasure

Encapsulate the logic used when creating an erasure coded pool into the
check_cluster_features helper.

check_cluster_features(CEPH_FEATURE_CRUSH_V2) is required for crush rule
create-erasure because it is expected that the erasure code plugin will
use indep instead of firstn and expect the V2 behavior and not the
legacy behavior.

The CEPH_FEATURE_CRUSH_V2 is added to CEPH_FEATURE_OSD_ERASURE_CODES
when an erasure coded pool is created. It is necessary because pool
won't function properly if given an indep ruleset that does not
implement the V2 behavior.

Signed-off-by: Loic Dachary <loic@dachary.org>