Describe method of updating images references and version when updating Istio

[i-chvets]

Description

istioctl binary is used to install Istio in Istio charms. Up until this update there was no method to specify which Istio images to use in deployment. After the above update there is a need for specification/description on where to look for image names and versions when updating Istio. I.e. where in upstream Istio project those can be found.

Configuration option for image that requires attention in the documentation:

  image-configuration:
    default: |
      pilot-image: 'pilot'  # values.pilot.image
      global-tag: '1.17.3'  # values.global.tag
      global-hub: 'docker.io/istio' # values.global.hub
      global-proxy-image: 'proxyv2' # values.global.proxy.image
      global-proxy-init-image: 'proxyv2' # values.global.proxy_init.image
      grpc-bootstrap-init: 'busybox:1.28'
    description: >
      YAML or JSON formatted input defining image configuration to use when installing the Istio control plane.
      For reference https://istio.io/v1.5/docs/reference/config/installation-options/
    type: string

[orfeas-k]

I will add that, for istio-pilot at least, it's not only image references but all the values set in the default config introduced here.

[NohaIhab]

For our airgapped effort, we'll use a static list for now due to time constraints and will revisit this to do it dynamically in the next CKF release.