stack/monitor: more robust httpd wrapper

This is a fixup of !1418
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Status Job ID Name Coverage
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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

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passed SlapOS.Theia.ResilienceTest-Master-ERP5

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed SlapOS.Eggs.UnitTest-Master.Python3

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