I've attached the requested logs.

As pjdc pointed out, the issue is that the hostnames are:

[prod-u1-sca] prod-u1-sca@wendigo:/srv/mojo/mojo-prod-u1-sca/trusty/production/charms$
juju status | grep dns
    dns-name: 10.35.128.11
    dns-name: 10.35.128.12
    dns-name: 10.35.128.13
[prod-u1-sca] prod-u1-sca@wendigo:/srv/mojo/mojo-prod-u1-sca/trusty/production/charms$
host 10.35.128.12
12.128.35.10.in-addr.arpa domain name pointer
12.128.35.10.instance.prodstack4.internal.
[prod-u1-sca] prod-u1-sca@wendigo:/srv/mojo/mojo-prod-u1-sca/trusty/production/charms$
host 10.35.128.13
13.128.35.10.in-addr.arpa domain name pointer
13.128.35.10.instance.prodstack4.internal.

and the rabbitmq charm (well,
charmhelpers.contrib.openstack.utils.get_hostname) assumes that you
can just do:

return result.split('.')[0]

with that - hence the rabbitmq hostname being set to 12 and 13 respectively:

$ juju run --service rabbitmq-server "cat /etc/rabbitmq/rabbitmq-env.conf"
- MachineId: "1"
  Stdout: 'RABBITMQ_NODENAME=rabbit@12'
  UnitId: rabbitmq-server/0
- MachineId: "2"
  Stdout: 'RABBITMQ_NODENAME=rabbit@13'
  UnitId: rabbitmq-server/1

I don't know the requirements on rabbitmq's nodename - I'm assuming
fqdn=True (the default) doesn't work?

nodename = get_hostname(ip_addr, fqdn=True)

otherwise, that would use less assumptions about the dns entry.

On Fri, May 1, 2015 at 5:20 PM, Edward Hope-Morley
<email address hidden> wrote:
> Michael, i've had a quick go at deploying lp:charms/trusty/rabbitmq-
> server using the local_provider and the steps you specify and I am not
> seeing this problem. I have also regularly deployed to openstack in the
> last few days and am not seeing an issue there either. Could i ask you
> to provide your rabbitmq juju logs e.g. /var/log/juju/unit-rabbitmq-
> server-0.log from all your units and also give me the service name of
> your deployed rabbitmq service and the hostnames of the nodes deployed
> to. Thanks.
>
> ** Changed in: rabbitmq-server (Juju Charms Collection)
>        Status: New => Incomplete
>
> --
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/1450699
>
> Title:
>   Cannot add more than one unit on openstack
>
> Status in rabbitmq-server package in Juju Charms Collection:
>   Incomplete
>
> Bug description:
>   This is possibly specific to the Canonical PS4 openstack environment
>   (or specifically, the dns resolution there), but it may be better if
>   rabbitmq defaulted to IP addresses for RABBITMQ_NODENAME.
>
>   Steps to reproduce:
>    1) Deploy rabbitmq-server into a fresh openstack environment
>    2) Add a second unit
>    3) Wait and check the status.
>
>   Expected result: new node has joined the cluster.
>   Actual result: cluster-relation-changed error because an invalid hostname of a single number has been set (eg. 'ERROR: epmd error for host "6": badarg (unknown POSIX error')
>
>   I initially saw it with r82 of the charm - so reproduced it again just
>   now there [1] (example plus full details of error), but also checked
>   and verified the same issue with r99 [2]
>
>   pjdc says that the bad hostname is likely from the reverse dns,
>
>   [1] http://paste.ubuntu.com/10958485/
>   [2] http://paste.ubuntu.com/10958620/
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/charms/+source/rabbitmq-server/+bug/1450699/+subscriptions