UI warning + document that changing deployment_option causes Base URLs to be cleared

When switching a service from hosted to self-managed, the Staging and Production Public Base URLs are cleared out.

When switching a service from self-managed to hosted, the Staging and Production Public Base URLs are set to a predetermined default value even if they had an existing value.

It can be somewhat confusing when dealing with on-premises installations where the lines are somewhat blurred between self-managed and hosted gateways. This can result in data loss when it's unclear which option should be used. Especially during upgrade from 2.5 to 2.6.

This action is only expected as a one time action for a Service - the use case is not to flip back and forth.

PROPOSED SOLUTION:

• Improve docs and specifically upgrade docs.
• In UI when activating a change in either direction provide a warning message "This action will have the effect to [clear out your Public Base URLs]/[set your Public Base URLs to a system generated] as well as to [delete the corresponding routes in OpenShift]/[create the corresponding routes in OpenShift]"
• clear out the input field when switching from hosted to self-managed
• make URLs field required in order to submit the form
• For APIs calls it is not possible to provide a warning - but the use case is generally for provisioning from a clean slate, so the existing stored state is not relevant. The upgrade scenario is a special case and this should be covered in docs.

Update (30/09/2021):
After discussing with UXD and Docs teams, we've agreed on:

• When switching from "APIcast self-managed" to "APIcast 3scale managed", Public URLs are not cleared, they are kept in memory and you recover them when switching back, until you submit the form.
• The warning messages will be updated to reflect this change:
  • From APIcast self-managed to APIcast 3scale managed message: "This action deletes the corresponding routes in OpenShift and reverts the Public Base URLs back to the latest value. Do you want to continue?"
  • From APIcast 3scale managed to APIcast self-managed  message: : "This action sets the Public Base URLs with values generated by the system and creates the corresponding routes in OpenShift. Do you want to continue?"

To enable the option for service mesh for testing purposes, see config/rolling_updates.yml.