Rename node.nodegroup to node.group

In the context of a node, its group has (at least today) only one
meaning, that is the node's node group. As such, we rename
node.nodegroup to just node.group.

Note: if we want to keep node in there, it should be at least
node_group, for consistency with the other node attributes.

Similarly, we rename the OpAddNode nodegroup attribute to group.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>