Make the node name separator in short link format configurable

[ipspace]

Based on a fun discussion with @ddutt:

• You cannot use the short link format if the hostnames contain '-'
• We could use some other character as the separator, for example A↔B↔C, but that would break almost any topology out there

However, if we define defaults.const.link.node_separator to be '-' and use that when splitting node names, users could use whatever separator they want ;)