Auto image picker

[XaverStiensmeier]

1. Image definitions in the configuration can now be regexes:

  masterInstance:
    type: de.NBI small
    image: Ubuntu 22\.04 LTS \(.*\)

2. fallbackOnOtherImage takes either a bool value or a regex. If True is given, the nearest image by name is used if the original image (can be regex) is not found among the active images. If a regex is given, the first regex match is used instead when the original image (can be regex) is not found among the active images.
3. Added documentation for this feature.

[jkrue]

lgtm