When rabbit_hosts parameter isn't set (service_default is used)
we are forming its value in that way: "$rabbit_host:$rabbit_port".
But when host or port or both aren't set (they are also service_default)
we can get rabbit_hosts set to "$rabbit_host:<SERVICE DEFAULT>"
or "<SERVICE DEFAULT>:$rabbit_port" or "<SERVICE DEFAULT>:<SERVICE DEFAULT>".
This patch fixes the issue. rabbit_hosts parameter will be set only if it's passed.
Change-Id: I9aed2b8a8718e0b0cfca3e25ef93fcff2ad8444a