caddy-frontend: Keep Server header intact

Server header might be changed by client facing caddy, for which caddy code
has been fixed to not transmit the Server header at all by default.
In ATS Server header has been disabled by configuration.
haproxy does not add Server header at all.
Thanks to improvement in the test backend, it's asserted that Server header
send by the backend is left intact.
14 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed Cloudooo.UnitTest-Master

00:06:41

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:42

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:44

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:40

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:32:31

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

21:56:05

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

22:57:58

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

08:38:19

passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:44

passed SlapOS.Eggs.UnitTest-Master.Python3

00:11:48

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:59

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:28:35

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:48:38

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

08:58:38