Read child resource operation

I realized one drawback with current API.
In order to fetch all sever groups, I need to get the list of sever group names first,
and for each group request the resource (N+1).

In order to reduce the number of requests necessary to fetch resources like
server groups, host, etc, I would suggest we add another generic operation that would be similar to read-children-names:

:read-children-resource(child-type=<name>)

It should return a list of resource representations that are children of <name>