Pass netinfo in rpcs

If a nic has a network field then encapsulate a network object in
netinfo slot for every rpc. This is needed to pass network info to
scripts managing nics (kvm-vif-bridge).

Introduce _BuildNetworkEnv().

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