Convert node_info rpc to new style result

This patch also does some cleanup and enforces valid results (with
proper type, i.e. int for memory/disk values) from remote node,
otherwise we handle the result as failure.

We do this so that we can remove custom processing in rpc.py which is
very old and doesn't play with the new style result (and has bit me by
modifying the result received from the remote node).

After this change, the node_info rpc becomes a normal one.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>