The pool spec has now a new property called "replicasPerFailureDomain"
which essentially represents the number of replicas to store in each
failure domain.
Assuming the failure domain is a datacenter (if the cluster is
stretched) then you will have 2 replicas per datacenter where each
replica ends up on a different host. This gives you a total of 4
replicas and for this, the "size" must be set to 4.
Closes: https://github.com/rook/rook/issues/5591
Signed-off-by: Sébastien Han <seb@redhat.com>