Chutney templates may need an option-fallback feature
In the parent, we want to split AssumeReachable into AssumeSelfReachable and AssumeOthersReachable. We will want Chutney authorities to set AssumeSelfReachable if that option exists, but AssumeReachable if it does not.
That's not something that the chutney config system can do right now AFAICT.
It might be simpler to do this with a magic bit of code specific to assumereachable as part of the chutney system, but I'm not sure.