name rabbitmq nodes with short hostnames (bz1119429)

RabbitMQ node names must be short names, not fqdn.  For example,
'node1.example.org' is invalid, but 'node1' is valid.  This strips off
the domain part of the hostname when creating the cluster node names.
Merge pull request #313 from jeckersb/rabbit_short_node_names

name rabbitmq nodes with short hostnames (bz1119429)