Pools in stretch clusters will all use the same crush rule that
is generated by the operator when configuring the cluster for
stretch mode, with replica 4 and two replicas per failure domain.
Pools cannot create new crush rules, so we require that replica be
4 when the pools is created, EC is not allowed, and no new
crush rule will be created.
Signed-off-by: Travis Nielsen <tnielsen@redhat.com>