validate a hub's `model-metadata-schema.json` config file

[elray1]

I think that the model-metadata-schema.json config file that a hub provides must satisfy the following:

• Is a valid json schema file
• Contains specification for a model_id OR both of team_abbr and model_abbr fields.
• Once we handle samples: if samples are being collected and a specification of sample dependencies is not specified at the hub level, we expect this to show up in the model metadata schema.