Provide a management operation to list the servers in a server group (domain mode)

In domain mode, an end user might want to start one or more servers by hand in a named server group. Obtaining the list of servers in a server group is not readily available: the "group" attribute is embedded in server-config, so we have to drill down into every /host=/server-config= combination to find out which servers belong to a specific server group.

It would improve the user experience to have an operation which shows which servers are in a given server group.