mod_cluster MCMP should internally use "lbgroup" instead of "domain"

Here is a requst to internally change MCPM to use lbgroup instead of domain but keep domain for compatibility reasons and a possible upgrade path.

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AS7-1721
We removed the notion of "domain" in context of mod_cluster since 1.1.x in order not to be confused with notion of domains used in AS7 for management perspective.

Currently, the XSD does not list out the LBGroup property but the CommonAttributes.java implementation parses the XML for "domain".

    String DOMAIN = "domain";