Implement network/nodegroup mappings

LUNetworkConnect() connects a network to a nodegroup and defines
its netparams (mode and link). Specifically:
 * Check if network already exists
 * Check if netparams are valid
 * Check if already connected to the same nodegroup
 * Produce errors if any conflicting IPs found
 * Update the 'networks' slot in NodeGroup config object
   with the network UUID and netparams requested. E.g.
   nodegroup_uuid {
     networks {
       network_uuid: {mode: 'bridged', link: 'br100'}
     }
   }

LUNetworkDisconnect() unmaps a network from a nodegroup:
 * Produce errors if an instance has a NIC with IP inside the network
 * Remove corresponding entry in 'networks' slot of NodeGroup object

Check for conflicting ips can be avoided with corresponding option.

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