Add JSON schema validation functionality

[thrix]

FMF should be able to consume a validation scheme to validate the metadata against.

[LecrisUT]

Although #161 is meged I think this can be improved

• define the local/remote path to the schema in .fmf/version, other metadata file or in a default file schema.json
• add the validation to the __init__ or add some helper classes/constructors like TreeValidated that automatically runs the validation if schema is found
• add a cli to detect/download all the json schema used for the current fmf tree

[LecrisUT]

I have been playing around implementing my own schema and considering the schemas in tmt:

• The schema validation should be done on the generated fmf tree, instead on the yaml file itself. The reason is that the +/- operators can easily break the validation. Validating against adjust would be tricky though
• The $ref is a bit ambiguous. I managed to get it to work if I specify the sub_schema.yaml instead of the $id, when working with pycharm. Maybe things will work better within the python code if we define appropriate schema stores