Fix topo devices with same short_name

Display devices in the topology map with the same short_name as
distinct nodes.

Fixes redmine #1822
Add as_canon Graphviz format

This is useful for debugging graph building
Update graph edge directions for device topo

Adjust the graph edges so that "Up" devices appear to the left and
"Down" devices appear to the right.

Fixes redmine #1824
Merge pull request #17 from candlerb/candlerb/1822

Topo fixes #1822 #1824