Serialize environment values - implement for all entities

[neykov]

Builds on #208

[grkvlt]

@neykov this is useful, although I don't much like the fact we have to do it explicitly everywhere. can't see a way of centralising it, unless we implement some kind of post-processor with a Function<T,String> toString as an option for every ConfigKey and a sensible default?

[grkvlt]

@neykov happy to merge as-is, anyway

[neykov]

That's very specific to bash environment serialization so doing a toString for config keys in general would need information on how the value would be used. I think it becomes too complex for little gain. In future I agree we could add object specific serialization when need arises.

[grkvlt]

@neykov OK, merging