offer simple way to express (un)supported keys

[ndeloof]

We need to replace #11 with a pluggable API for implementations to express keys they don't support.

v3 broke the v2 approach to define services options as a list of fields, that can be easily white|blacklisted as []string. We need a way to filter an actual yaml tree.

Proposal:
introduce option for implementation to declare a whitelist (or blacklist) of yaml paths. Check configDict map[string]interface{} to only/not include elements matching such paths

It doesn't seem there's a standard syntax to express paths in yaml tree, nothing comparable to xpath. Assuming we can use a comparable syntax, we could define a blacklist like this:

• services/*/deploy/placement
• services/*/volumesFrom

[hangyan]

python seems have a library to parse yaml tree: https://pypi.org/project/yamlpath/. But still there is no standard way to define the syntax, we can choose one and implement it in golang

[ndeloof]

This is now feasible relying on tree.Path syntax