Setup CI for the Release Drafter configuration

[oleg-nenashev]

I have already broken Release Drafter in Jenkins few times. It would be great to have some kind of CI offered for it.

• There is a schema for the Release Drafter configuration (Document the Release Drafter JSON Schema release-drafter/release-drafter#311)
• There is a Jenkins Pipeline step which invokes the schema validation

[PierreBtz]

This is blocked by release-drafter/release-drafter#600, the provided schema was not following the draft 04 spec, therefore no validation could be done :/

[PierreBtz]

@oleg-nenashev I started to write something to address the validation in #39 . It's a pretty basic bash script, I was just curious what to expect from a json/yaml validator... Let me know if you had something else in mind I will drop this :)