caddy-frontend: Add promise for kedifa auth generation

Instead of lying in tests and checking for NotReadyYet in the connection
parameteres use promise to assert that generation of KeDiFa URLs is really
ready.
3 jobs for feature/caddy-frontend-cleanup-202207 in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-py3-luke/feature/caddy-frontend-cleanup-202207

02:47:35

failed SlapOS.SoftwareReleases.IntegrationTest-py3-luke/feature/caddy-frontend-cleanup-202207

02:38:18

failed SlapOS.SoftwareReleases.IntegrationTest-py3-luke/feature/caddy-frontend-cleanup-202207

02:50:55