Support APICast 2.x with System 2.x-1 for more flexible upgrades

Sometimes it is desirable to upgrade a self managed gateway before upgrading 3scale (system).

Generally APIcast has been backwards compatible but this is a request to officially support this configuration. 

It's possible that QE testing of the 3scale operator upgrade process already covers the testing required here but we should discuss this in greater detail.

Scenarios to cover:

• 3scale w/ self managed APIcast - Customer encounters a production affecting bug in APIcast that is fixed in a newer version. In order to upgrade APIcast they will need to upgrade system first which is a much more involved process. Thus resulting in a longer time to recovery.
• 3scale w/ embedded APIcasts - During the upgrade process, the rollout process may cause APIcast to be deployed before System resulting in a small downtime if the new version of APIcast is not compatible with the previous version of System. No known occurrences of this have occurred but this scenario came up in our discussion of the first scenario. The edge case would be if APIcast managed to start but failed to load configuration or policies for some unforeseen reason. 

If/when this support is added, the supported configurations page should be updated to reflect this, giving Support and our customers more supported avenues to resolve issues.