Implement LUNetworkQuery

Summarily list all existing networks
Supply detailed info for every existing network
 - List used/free IPs
 - List instances with NICs assigned to the corresponding network
 - List NIC index and IP for the above instances

Implement complementary config methods for retrieving networks.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Iustin Pop <iustin@google.com>