caddy-frontend: Re-introduce promise plugin assertions

Promise plugins are important part of the instantiated configuration, thus
have to be checked for changes.
8 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:41

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:47:13

failed SlapOS.SoftwareReleases.IntegrationTest-Master

06:00:53

passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:27

passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:34

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

03:46:24

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:49:00

failed SlapOS.SoftwareReleases.IntegrationTest-Master

07:05:56