caddy-frontend: Solve ATS configuration asymmetry

Accessing http with path starting with /HTTPS/ would result with 503 responses
from ATS itself.

Test is covering the problem and also asserting that fix does not damage https
access with path starting with /HTTP/
13 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:29

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:21:49

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

06:35:44

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:36:52

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

09:45:59

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:41

passed SlapOS.SlapRunner.ResilienceTest-Master

00:21:57

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:25:51

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:27:20

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

24:08:02

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:49:51

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

12:00:31

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

11:32:20