Adds option to read topology from a file in floodlightTopologyExporter.py

This commit adds the option of reading the topology from a file in
floodlightTopologyExporter.py. However, the set of nodes is still retrieved
from the controller. This is useful to force a physical topology that is not
known by the controller.
Minor change: improving documentation for SDNHop class
Updating sdnPSS dependency to 46f64256cc74614f56dab8aa6a0e74023dc6c3ba