Everything worked OK with a 3 node cluster in a openstack cloud without dns support for the instances

Here is the evidence:

environment: openstack-freyes
machines:
  "0":
    agent-state: started
    agent-version: 1.20.14
    dns-name: 10.55.61.143
    instance-id: cfdbd16e-6ce8-4996-a552-105b0d905a32
    instance-state: ACTIVE
    series: trusty
    hardware: arch=amd64 cpu-cores=1 mem=1024M root-disk=10240M
    state-server-member-status: has-vote
  "1":
    agent-state: started
    agent-version: 1.20.14
    dns-name: 10.55.61.145
    instance-id: a96f334c-31fb-495d-a4ca-96e063f4b672
    instance-state: ACTIVE
    series: trusty
    hardware: arch=amd64 cpu-cores=1 mem=1024M root-disk=10240M
  "2":
    agent-state: started
    agent-version: 1.20.14
    dns-name: 10.55.61.146
    instance-id: fe3b66a5-4a2b-4cf1-9f03-cccbf56c7356
    instance-state: ACTIVE
    series: trusty
    hardware: arch=amd64 cpu-cores=1 mem=1024M root-disk=10240M
  "3":
    agent-state: started
    agent-version: 1.20.14
    dns-name: 10.55.61.147
    instance-id: 7a61ee9e-acd6-44a5-911d-61c1cb732f52
    instance-state: ACTIVE
    series: trusty
    hardware: arch=amd64 cpu-cores=1 mem=1024M root-disk=10240M
services:
  rabbitmq-server:
    charm: local:trusty/rabbitmq-server-128
    exposed: false
    relations:
      cluster:
      - rabbitmq-server
    units:
      rabbitmq-server/0:
        agent-state: started
        agent-version: 1.20.14
        machine: "1"
        open-ports:
        - 5672/tcp
        public-address: 10.55.61.145
      rabbitmq-server/1:
        agent-state: started
        agent-version: 1.20.14
        machine: "2"
        open-ports:
        - 5672/tcp
        public-address: 10.55.61.146
      rabbitmq-server/2:
        agent-state: started
        agent-version: 1.20.14
        machine: "3"
        open-ports:
        - 5672/tcp
        public-address: 10.55.61.147
$ for i in 0 1 2; do juju ssh rabbitmq-server/$i 'sudo rabbitmqctl cluster_status'; done
Cluster status of node 'rabbit@juju-openstack-freyes-machine-1' ...
[{nodes,[{disc,['rabbit@juju-openstack-freyes-machine-1',
                'rabbit@juju-openstack-freyes-machine-2',
                'rabbit@juju-openstack-freyes-machine-3']}]},
 {running_nodes,['rabbit@juju-openstack-freyes-machine-3',
                 'rabbit@juju-openstack-freyes-machine-2',
                 'rabbit@juju-openstack-freyes-machine-1']},
 {partitions,[]}]
...done.
Connection to 10.55.61.145 closed.
Warning: Permanently added '10.55.61.146' (ECDSA) to the list of known hosts.
Cluster status of node 'rabbit@juju-openstack-freyes-machine-2' ...
[{nodes,[{disc,['rabbit@juju-openstack-freyes-machine-1',
                'rabbit@juju-openstack-freyes-machine-2',
                'rabbit@juju-openstack-freyes-machine-3']}]},
 {running_nodes,['rabbit@juju-openstack-freyes-machine-3',
                 'rabbit@juju-openstack-freyes-machine-1',
                 'rabbit@juju-openstack-freyes-machine-2']},
 {partitions,[]}]
...done.
Connection to 10.55.61.146 closed.
Warning: Permanently added '10.55.61.147' (ECDSA) to the list of known hosts.
Cluster status of node 'rabbit@juju-openstack-freyes-machine-3' ...
[{nodes,[{disc,['rabbit@juju-openstack-freyes-machine-1',
                'rabbit@juju-openstack-freyes-machine-2',
                'rabbit@juju-openstack-freyes-machine-3']}]},
 {running_nodes,['rabbit@juju-openstack-freyes-machine-1',
                 'rabbit@juju-openstack-freyes-machine-2',
                 'rabbit@juju-openstack-freyes-machine-3']},
 {partitions,[]}]
...done.
Connection to 10.55.61.147 closed.