Create a layer for mod_cluster subsystem

This should probably look like as follows:

• the base layer dependency needs to be web-server
• that however doesn't provision ajp listener, so the default feature-group-spec should use the default connector
• the ha profile (unsecured-basic-ha-profile) which provides the ajp feature should then update the reference in the subsystem
• the layer needs to provision standard sockets - the advertise address/iface