Standardize image names and source URLs

Created by: mbuechse

We want users to be able to upload their own images to Glance, so we mandate that this must be possible (with some quota).

We want to standardize image names and source URLs with the intention that the CSP can provide any subset of these images at will; AND if an image with a name from that list is provided, it has to be from the given source URL. The list should be given by a yaml file, whose schema could be a subset of the one for the image-manager yaml files.

The only mandatory image should be the kubernetes-capi images. These could be listed in the yaml file as well and marked mandatory.

Work on this issue can be done inside https://github.com/SovereignCloudStack/standards/pull/314 -- at least the applicable parts from that PR should be reused.

Definition of done:

• A new standard is written that covers image upload and standard names/URLs (given via a yaml file that is regarded as an annex).
• The yaml file lists the mandatory kubernetes-capi images.
• From the existing image metadata standard the section "Standard images" is removed.
• A test script has been written for the new standard, and it has been added to https://github.com/SovereignCloudStack/standards/blob/issue/267/Tests/testing/scs-compatible-test.yaml.
• The new standard has been added to https://github.com/SovereignCloudStack/standards/issues/317