Fix RAPI node modify client and server calls

rapi.client.ModifyNode accepts a "group" and not a "node" param.
(this bug is invisible but still not nice)

rlib2.R_2_nodes_name_modify submits the opcode with instance_name rather
than node_name as a param. This would break the call.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>