topo: make new minimal Graph object a Graph, not a DiGraph
Fixes another Graph regression relative to NetworkX.
RipL broke because the NetworkX Graph object that was used previously
for topologies is an undirected graph:
import networkx as nx
There is a separate DiGraph object in NetworkX for directed behavior.
The minimal replacement previously implemented DiGraph behavior.
from mininet.topo import Graph
This commit restores undirected graph behavior.