New feature: cluster scope for pcp_stop_pgpool utility command

New feature: cluster scope for pcp_stop_pgpool utility command

Make pcp_stop_pgpool utility to terminate (optional) all the pgpool nodes in the
watchdog cluster
For that purpose added  "--scope=cluster/local" option in the pcp_stop_pgpool
utility to set the scope of the command. The scope could be either
'local' or 'cluster'

Along with that, the commit adds the infrastructure in watchdog for broadcasting
the custom commands over the watchdog network. This will make it easier to
implement the --scope=cluster for other PCP utilities.