HASingletonElectionPolicy should only contain methods that actually called from
the HASingletonSupport/HASingletonController classes so that they can inject
attributes to the election policy that users don't have to specify in the ha singleton
election policy bean definition.Therefore:

/**

• Sets the preferred master node. As long as the preferred master node
• presents in the cluster, it will be always selected as master node,
• no matter what the election policy is.
• @param node String format of ip_address:port_number
  */
  void setPreferredMaster(String node);

String getPreferredMaster();

The methods above should not live in HASingletonElectionPolicy, but instead on its
own policy impl class..