caddy-frontend: Cleanup var/auth-ready

Slaves can disapper during handling of authentication, which will result
with prepared reservation, but with any chance to mark that it's ready.
10 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

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passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

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passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

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passed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.KVM.ResilienceTest-Master

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passed SlapOS.SlapRunner.ResilienceTest-Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

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