Initial stab at monitoring_check::cluster

initial stab at cluster check, just need to know if i'm going in right direction?
this is valid, i'm looking into how to create and send event into MQ but here's another idea - why not implement "lockable" checks on sensu client level? somewhere here: https://github.com/sensu/sensu/blob/master/lib/sensu/client.rb#L97

as long as we have it forked anyway i think this is valid functionality and doesn't require to expose transport and redis settings to check process