Have os-image-server expose OS images produced by diskimage-builder

What does this MR do and why?

This MR allows to use images produced as OCI artifacts in the diskimage-builder project in os-image-server unit.

This MR introduces new entries in cluster map values:

• os_image_server.external_hostname: similar as other external_hostname configuration
• os_images: which it used for configuring user provided images.

os_images takes as parameter a map which provides data about images to be served (as described in related helm chart).
If user don't provide images, ones from diskimage-builder project are used as default.

Note: default configuration is not directly set up in core cluster.os_images in values.yaml as I would be painful to override such configuration directly. Alternatively cluster.os_images is empty and default values are used (_internal.defaultOsImages). If cluster.os_images is not empty, it is used directly and default values are dropped.

Related reference(s)

Closes #377 (closed)

Test

Unit os-image-sever can be manually enabled any management cluster deployment to test everything is going right.