Adapt the messaging subsystem clustering config to current EAP best practice

The best practice HA config for the messaging-activemq subsystem has evolved in the last year and what ends up in standalone-openshift.xml has fallen behind a bit:

1) The jgroups-channel attribute has been deprecated, replaced by jgroups-cluster. This attribute is not meant to be a reference to a channel in the jgroups subsystem, rather it is higher level identifier for the nodes meant to form a service-specific cluster.

2) The default name should be "activemq-cluster" rather than "ee". The nodes forming a cluster are doing so for purposes of making activemq HA.

This needs some examination in terms of impact on other layers. We need to check when jgroups-cluster came into existence. Perhaps this is only applicable to EAP CD and later, in which case we can handle this in a CD specific module for now, and only bring it to a shared module when we drop support for 7.0/7.1.

Point 2) above could go in a shared module today, but probably isn't worth changing on its own.

(Edit: the jgroups-cluster attribute is only available starting with EAP CD 12, so if we don't drop all this altogether it can only be used for the EAP CD images.)