Add YAML interfaces in line with idiomatic go

[kerneltime]

From #271 I think we should rather implement here the YAML unmarshaller interface: https://github.com/go-yaml/yaml/blob/v2.2.4/yaml.go#L32

[fredbi]

ok we have adopted a pattern to have pkg/model objects wired as YAML documents. We just need now to make this approach consistent for all metadata objects.