Replace nodegroups' PartialNic by PartialNicParams

This fixes a bug that corrups the config when one adds a
network to a node group. I wil soon extend QA by some commands
that would have detected this.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>