Messaging BroadcastGroupDefinition & DiscoveryGroupDefinition fixes

Several bugs that manifest when trying to create or edit broadcast groups or discovery groups:

• BroadcastGroupDefinition.CONNECTOR_REFS is non-required attribute, while both its specializations JGroupsBroadcastGroupDefinition.CONNECTOR_REFS and `SocketBroadcastGroupDefinition.CONNECTOR_REFS` are required. BroadcastGroupDefinition.CONNECTOR_REFS should be required too.
• BroadcastGroupWriteAttributeHandler.SOCKET_INSTANCE uses attribute list from JGroupsBroadcastGroupDefinition rather than from SocketBroadcastGroupDefinition.
• A NPE is thrown from SocketBroadcastGroupAdd.createBroadcastGroupConfiguration() when the provided socket binding doesn't have multicast address configured. Show a contextual error message instead. NPE is from the line:

  final String groupAddress = socketBinding.getMulticastAddress().getHostAddress();

• The same issue as above in SocketDiscoveryGroupAdd class.
• Running /subsystem=messaging-activemq/server=default/broadcast-group=bg-group1:undefine-attribute(name=socket-binding) throws a NPE from TranslatedWriteAttributeHandler}} - from line

  OperationStepHandler writeAttributeHandler = context.getRootResourceRegistration().getAttributeAccess(targetAddress, op.get(ModelDescriptionConstants.NAME).asString()).getWriteHandler();

  +
  Note the broadcast-group resource is a "shallow" resource that has two "specializations": socket-broadcast-group and jgroups-broadcast-group. The "bg-group1" is actually a jgroups-broadcast-group, which ignores the socket-binding attribute. The handler should probably skip the operation if the attribute belongs to ignored attributes.

• Same issue in discovery group.