XXX caddy-frontend: Switch from Caddy to Haproxy

TODO:

 * generate configuration for haproxy
 * reimplement caddy-backend-url-validator.in
 * merge frontend and backend haproxy into one process
2 jobs for feature/caddy-frontend-haproxy in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/caddy-frontend-haproxy

06:02:20

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/caddy-frontend-haproxy

10:28:45