Generate spanning tree for bootstrapping -- Python 3 porting

[congwang09]

Create a separate set of code to read the SDX topology from manifest, use NetworkX to determine a spanning tree (steiner tree), output to a new topology file, which will be distributed to all the SDX nodes for control plane boot strapping.

Try Python 3 for this part. Try if NetworkX will work on Python 3. This can be a first step for Python 3 porting.