Add a generic query facility

To find out, which e.g. jdbc drivers are deployed on a server group, I have to visit each managed server and query it. This is cumbersome

• first the server group should have the same driver for all servers, so asking the server group should be enough
• as the server group does not even know its connected managed servers (there is no :list-servers) operation, I need to potentially loop over the whole domain including tons of remote HC in order to query for e.g. the jdbc drivers or other stuff

There should be a mechanism to fire queries at which then get relayed to the HCs and the answers combined and returned.

Still I don't get why if a server-group is meant to contain an identical configuration for all its managed servers (besides VM params, ports and IPs) it does not just have "proxy methods" like :installed-drivers-list or :server-list etc.