pgraph: Remove NewVertex and NewEdge methods and fix examples

Since the pgraph graph can store arbitrary pointers, we don't need a
special method to create the vertices or edges as long as they implement
the String() string method. This cleans up the library and some of the
examples which I let rot previously.

test/shell/libmgmt-change1.go

@@ -68,9 +68,8 @@ func (obj *MyGAPI) Graph() (*pgraph.Graph, error) {
 			MetaParams: metaparams,
 		},
 	}
-	v := pgraph.NewVertex(n0)
+	g.AddVertex(n0)
 
-	g.AddVertex(v)
 	//g, err := config.NewGraphFromConfig(obj.data.Hostname, obj.data.World, obj.data.Noop)
 	return g, nil
 }