Toolbox to support updating only mapping rules and metrics/methods

Currently toolbox OpenAPI import operation does not support App ID / App Key security, and multiple backends.

Hence, when updating the existing service with the above settings they will be overwritten.

The request is to provide a command argument that will permit only touching the mapping rules and metrics, but not the rest of the API configuration.
This will cover the use case when we only add new endpoints to an already existing (and configured in 3scale) API.