FOI: YAML schema/validation

[yarikoptic]

I was looking around for ways to specify schema for YAML so it could be validated... still searching for an optimal tool, but it might be that we end up using not a YAML-based schema for YAML but rather a tool providing a (more flexible/concise) schema specification, such as voluptuous or schema (for python).

Re voluptuous -- there is even a pytest extension for it: https://github.com/F-Secure/pytest-voluptuous which looks neat. We have already some home-brewed helpers along those lines but I thought it might come handy

@kyleam is using json schema validation in pyout, he might provide a bit of direct "experienced" feedback ;-)

[yarikoptic]

FWIW, elasticluster uses the https://github.com/keleshev/schema mentioned above