Update the "onos-service" command:
 - add a mechanism to print the usage.
 - If the target is "--cell", the service command is executed on each
   node in the current ONOS cell.

Usage:     onos-service <TARGET> [COMMAND]
           onos-service [-h | --help]
Options:
    TARGET          The target of the command
    COMMAND         The command to execute. Default value is 'status'
    [-h | --help]   Print this help

TARGET:  <hostname | --cell>
      hostname        Execute on the specified host name
        --cell        Execute on the current ONOS cell

COMMAND: [start|stop|restart|status]
1 file changed