Add option for specifying Node.Mode

- Added a command-line option to the Client to specify the mode to be
  sent to the master node (either 'normal' or 'exclusive' which
  correspond to values in hudson.model.Node.Mode)
- Added a specific OptionHandler to parse those
- Added another @QueryParameter to hudson.plugins.swarm.PluginImpl to
  make sure the mode gets correctly applied
- Added Mode as a parameter to the SwarmSlave constructor that's being
  called from PluginImpl