AS7-4894 HA Singleton deployer for applications

OK - here's a 2nd draft...

I've made the following changes:
* The singleton service now wraps a specific Service&lt;DeploymentUnitPhase&gt;, instead of the whole Service&lt;DeploymentUnit&gt;.
* A new "singleton-deployment" subsystem that allows the user to configure 1 or more deployment policies.  This addresses the concern that the management API cannot determine the possible values for deployment-policy.
* You can now configure the cache-container and singleton election policy for each deployment-policy.
* SingletonService will rollback the deployment installation if deployment fails
* DeploymentUnitPhaseServiceBuilders are now installed as services - replacing the ServiceLoader mechanism used previously.  Consequently, the jboss-as-server module no longer requires an optional dependency on the jboss-as-clustering-singleton-deployment module.