HASingleton custom election policy isn't called with only one member

A custom HASingleton election policy is not called when there is only one member in the cluster.

org.jboss.ha.framework.server.HASingletonImpl.election() {
List candidates = getElectionCandidates();
if ((candidates == null) || (candidates.isEmpty()))
return null;
return ((candidates.size() == 1) ? (ClusterNode)candidates.get(0) : this.electionPolicy.elect(candidates));
}

The hard-coded check for size == 1 should be removed, and the custom policy should always be called.

This bug prevents policies that do not start HASingletons if a quorum is not reached.