Updates graph build API endpoint - timestamps are now passed in the JSON body and not in the URL query string

Moves SDN controller and SFEMC port numbers out of implementation - passed to CLMC as environment variables, default 8080 port used if not passed

Modified the build_network_graph API endpoint to read the sr-cluster mappings from a file - temporary solution

Added a function to generate the network topology given the switch-to-switch links, switch IP addresses and cluster-to-router mapping; changed ComputeNode to Cluster; added Switch nodes; modified tests