Update schema and tests for mediaType validation

[sajayantony]

Ideally it would be better to make the mediaType a required property. But given that we want to be backward compatible, the check is left to the client to ensure that layers/config doesn't appear with the manifests then mediaType is not present.

This PR doesn't attempt to fix that requirement in the schema and fixes only tests to have a valid mediaType.

-- Signed-off-by: Sajay Antony [email protected]