introduce OS image selectors

Related MR: automatic OS image activation (capo) (sylva-projects/sylva-core#2013 - closed)

This needs to merge on top of !2 (merged) and depends on sylva-library!44 (merged)

With this MR, sync-openstack-images will only produce an nginx StatefulSet and Ingress for OS images that match one of the OS image selectors specified in .os_image_selectors.

This MR can be introduced without breaking things around, since the default value of .os_image_selectors is one "match all" selector. Said otherwise the default behavior once this MR merges is to use all images from os_images like was the case before this MR.

sylva-core MR: sylva-projects/sylva-core!3661 (closed)