In clusters where only two datacenters (or similar failure domains)
are available, a different mon and osd approach is needed to deal
with the network partitions or some other reason for one of the failure
domains going down. The Ceph stretched cluster makes the mons aware
of the failure domains by configuring one as the arbiter in a third
zone, while keeping two replicas of the data in each of the data
zoens.
Signed-off-by: Travis Nielsen <tnielsen@redhat.com>