rename tracingConfigRef to tracingConfigSecretRef in 3scale Operator

The `.spec.openTracing.tracingConfigRef` was renamed to `.spec.openTracing.tracingConfigSecretRef` in https://issues.redhat.com/browse/THREESCALE-7531 as a more clear name to indicate that what needs to be provided for the config is a Secret in APIcast operator, but not in 3scale Operator. This issue is about resolving the inconsistency and renaming it also in the 3scale operator.