Expose service to display OS images changelog in elemental-operator controllers

[davidcassany]

The changelog data is present in ManagedOSVersionChannel images.
The changelog data should be extracted and exposed via HTTP to users.

[fgiudici]

Data is expected to be present in ManagedOSVersionChannel images (one file per OS image delivered through the channel, at path /changelog/${managedosversion name}.updates.log).
The idea is to expose the changelog data through the Elemental (Rancher) service URL, something like:
${RANCHER_URL}/elemental/changelog/...
This should be achieved extracting the changelog data from the channel image, publishing it on http via k8s resources (Pod + service) and exposing it via the Elemental/Rancher UI URL.

[fgiudici]

Initial PoC drafted in #891 .

After discussion and investigation, the path we envision to properly implement the feature in the operator involves:

• embed the service to expose changelog data in the ManagedOSVersionChannel CRD and controller
  • changelog data to be extracted from the ManagedOSVersionChannel image and exposed via HTTP through a worker Pod
  • the ManagedOSVersionChannel controller can be improved moving the ManagedOSVersion data extraction and resources creation to the same worker Pod
  • the Elemental Operator code can be further improved developing a generic workload Pod feature to be used by the SeedImage and MaangedOSVersionChannel controllers
• only one worked Pod per ManagedOSVersionChannel should be able to serve all the available changelogs (one for each ManagedOSVersion), using under the hood one httpfy instance
• the changelog data should be extracted and published as soon as a ManagedOSVersionChannel resource is created