split snmp-graph.properties

I suggest that some sort of "split" configuration be used to aid upgrades. By
this, I mean that some configuration files (snmp-graph.properties for example)
should check two locations - the "base" distributed file, and a second
"customised" file. Items in the second override the base file, if they
conflict. Otherwise the result is the union of those two files. This will not
work for all config files (the semantics aren't there), but for the ones it will
work on, it would significantly decrease the burden of upgrading. New
distributed files can be dropped into place over the old, and any customisations
will still exist without having to manually merge them together.