This switches the code to use networkx for the digraph implementation.
Note that the old implementation specifically isn't removed in this
change -- for review clarity. It will be replaced by a base class
that defines things properly to the API described below.
Plugins return a node object with three functions
get_name() : return the unique name of this node
get_nodes() : return a list of nodes for insertion into the graph.
Usually this is just "self". Some special things like partitioning
add extra nodes at this point, however.
get_edges() : return a tuple of two lists; edges_from and edges_to
As you would expect the first is a list of node names that points to
us, and the second is a list of node names we point to. Usually
this is only populated as ([self.base],[]) -- i.e. our "base" node
points to us. Some plugins, such as mounting, create links both to
and from themselves, however.
Plugins have been updated, some test cases added (error cases
specifically)
Change-Id: Ic5a61365ef0132476b11bdbf1dd96885e91c3cb6
