UserDomain can't be added to app settings via Arm or ADO

> Please provide us with the following information:
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### This issue is for a: (mark with an `x`)
```
- [x ] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
>
deploy to azure using arm template with userdomain defined as a parameter

### Any log messages given by the failure
>The data is not valid (Invalid or not allowed environment key name: UserDomain

### Expected/desired behavior
> add userdomain as an app setting to web app

### OS and Version?
> Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

### Versions
>

### Mention any other details that might be useful
I tried deploying using custom template and azure devops, both failed with the same error. removing the param and app setting from the template deploys the app successfully.
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Beginne damit, die Bereitstellung mit einem ARM template und Azure DevOps zu reproduzieren, wobei UserDomain als Parameter und app setting verwendet wird. Untersuche den Validierungspfad, der den environment key ablehnt; abgeschlossen ist die Aufgabe, wenn das app setting über beide gemeldeten Pfade ohne den invalid-key-Fehler erfolgreich bereitgestellt wird.

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