Set sample_default for rpc_zmq_host
Set sample_default for rpc_zmq_host

oslo-config-generator will use the sample_default for the sample
config file rather than the hostname of the system it happens to
be running on.

Change-Id: I9c7bf24cfe5c66155d00d9a1b390894d1ccb6bd8

redhat-openstack/oslo.messaging,isyippee/oslo.messaging,apporc/oslo.messaging,magic0704/oslo.messaging,zhurongze/oslo.messaging,dukhlov/oslo.messaging,stevei101/oslo.messaging,zhurongze/oslo.messaging,stevei101/oslo.messaging,apporc/oslo.messaging,hkumarmk/oslo.messaging,eayunstack/oslo.messaging,redhat-openstack/oslo.messaging,dims/oslo.messaging,hkumarmk/oslo.messaging,viggates/oslo.messaging,magic0704/oslo.messaging,ozamiatin/oslo.messaging,dukhlov/oslo.messaging,dims/oslo.messaging,ozamiatin/oslo.messaging,isyippee/oslo.messaging