Rename JGroups-related attributes

Based on Paul feedback in https://github.com/wildfly/wildfly/pull/8221#issuecomment-145013596:

You may want to consider renaming these attributes, however, since the names are now a little confusing. For example, jgroups-stack is perhaps better described as jgroups-channel-factory, since it can specify either a stack name or a channel name. jgroups-channel is also confusing in this context, since this specifies either the cluster name, in the case where jgroups-stack specifies a stack; or the fork name, in the case where the jgroups-stack specifies a channel name. Perhaps jgroups-cluster-name is a better choice?