Minimum line length setting for topo graphs

Add a setting for the minimum edge length. This makes it possible to space the nodes out a bit and have labels at opposite ends of an edge less likely to overlap.

This relates to redmine #1821

For rebuilding the whole topology graph you must add flags like `-n -m 2` into your crontab; and if you regenerate the graphs via the GUI then at the moment it reverts to defaults.