Document how to add custom policies to Self-Managed APIcast without access to 3scale

The On-premises documentation has the chapter: 4.4.3. Adding custom policies to APIcast on another OpenShift Container Platform which describes the procedure to add custom policies to APIcast self-managed. This requires access to the 3scale deployment to add policies to the APIcast built-in gateways. 

There are some scenarios where custom policies can't be added to the built-in gateways.

The procedure to add custom policies in such scenarios is the same as described in our SaaS documentation: 4.5. Adding custom policies to APIcast.

It would be helpful to add a chapter in the On-premises documentation for those use cases and point to the SaaS documentation or describe the same procedure as SaaS. 

Current behaviour

Adding custom policies to a self-managed APIcast is done through a process that requires the user to add those custom policies to the 3scale hosted/managed APIcast. The guide describes this process but does not consider the scenario where the user performing these steps might not have direct access or permissions to modify the 3scale hosted/managed APIcasts such as in a RHOAM deployment.

Expected behaviour

Adding custom policies to a self-managed APIcast is done through a process that requires the user to add those custom policies to the 3scale hosted/managed APIcast. The guide should describe this process including the scenario where a user does not have permissions or access to the 3scale hosted/managed APIcast instances. This could be because those APIcast instances are hosted on RHOAM or simply becasue the user doesn't have administrative access to the hosted APIcast deployment objects in 3scale On-premise. Both scenarios result in the same steps required to solve the issue and is already described in detail in the SaaS documentation