Context

We use rolling updates in porta to enable/disable features for container builds
But on-premises builds are using volume mounts to set those values. Which sometimes lead in desynchronization and mistakes. See https://github.com/3scale/3scale-operator/pull/117#issuecomment-478922952

Goal

• remove the templates volume mounts, having one default from the code. Less maintenance, one source of truth
• defaults can be merged with existing extras configs.
• document how to enable/disable certain features on-premises

Dev notes

The ideal solution would be:

• defaults are read from the config/*.yml
• custom are read from /opt/system/extra-configs/*.yml and merged to the defaults.