Define how context builders should handle missing service instance info in "get context" request
 - respond with error (400) if any of the service instance parameters is missing
 - respond with error (400) only for parameters that are absolutely required in order to get data; ignore parameter that are not really needed

AAI Context Builder - currently fails if any of the parameters is missing 

SDC Context Builder - currently fails if service instance id, model version id or model invariant id are missing; ignores customer id and service type

Ensure all context builders follow the same design.