ProxyConfigPromote CR - improve config to stage promotion

What

Product in 3scale can be promoted from base product to stage env, then to production env.
When we are promoting product from stage to production via CR, we always check if there's a difference and if no difference between existing stage and prod is discovered, the CR status is failed with message that no promotion happened due to no changes detected between envs.

When promoting base to stage, the check is missing and we are bumping whatever is in base to stage regardless.

This is a bit confusing behaviour because of it's inconsistency.

We should ensure that base product is confirmed to be different before removing the proxyConfigPromote CR and if no difference is found, a status message should be populated like in case of the production bump and CR should not be deleted.

How
This can be achieved by fetching the currentConfig here: https://github.com/3scale/3scale-operator/blob/master/controllers/capabilities/proxyconfigpromote_controller.go#L150 then bump product to stage, then confirm that the latestStageProxy config is different than the currentConfig fetched before. If no difference is found between versions of base and stage log a status error message for the customers.