Fix the name/label bug in add_node_to_graph

I could not understand the logic behind the design decision stated in the comment: 

> Node name can vary from label but must always be same for the same node label

pydot supports setting the name (key) and label (caption) to  different values. So I think the correct way to implement this function is to separate these two.

The reason I made this change was that in `rqt_graph` I wanted to change the label of noded to include more information. With the old implementation, changing the label independent of node name is impossible.