Getting rid of group.node_data

[ipspace]

Now that we'll finally handle attributes of all most objects in netlab topology, we could get rid of group.node_data and just define node attributes within a group dictionary.

The attributes would be validated using a combination of [ group, node ] attributes + providers (so we would know they're valid) and everything but group attributes would be copied into node data.

As a transition path, the early group data structure validation would copy everything in node_data back into the group dictionary and pop node_data dictionary. After a long while, we would make node_data attribute invalid.

Thoughts @ssasso @jbemmel?

[jbemmel]

Makes sense, should be a low impact change. Probably want to include a deprecation warning for node_data

[ipspace]

Implemented in b78ca80