DSL: Application can have attributes which can be changed only in runtime

Use can obtain final json description of environment which will contain all attributes exposed by applications. Technically if user change attribute and send this json back to Murano it can break something. We need to provide a way for app writer to prevent attribute change by user exposing it in json at the same time. I propose to have some "read-only" or "protected" attributes in application definition. Murano API will validate attribute change in uploaded json. If "protected" attribute was changed by user, Murano API will reject the request.