This command should be used to join this agent to an existing Fabric cluster.

By specifying zookeeper URL, this agent will be registered as a root agent, in the existing zookeeper instance

 e.g. fabric:join myhost:2181

 Note that root agents should have different names (karaf name) or they'll override each others data.

 You can change the name, by changing karaf.name property in ${KARAF_HOME}/etc/system.properties
