[WFLY-1430] Provide a cluster subsystem 

This PR is an implementation of a cluster subsystem, as described in https://issues.jboss.org/browse/WFLY-1430

In this PR, the subsystem is included as an extension and a (default) configuration supplied in standalone-ha.xml so that the subsystem is activated at boot time. Changes to this arrangement (i.e. allow users to install the subsystem by hand as and when desired as opposed to including in the standard clustered configurations) is up for discussion.