Make image source configurable

Currently, the images used for OpenStack services and such are hardcoded into the templates.

For ~~#66 (closed), #67 (closed) and~~ #101 (closed), offline installations and local mirroring of images, we need a proper way to configure the image source safely.

Edited Apr 26, 2021 by Jonas Schäfer

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information