change variable name to a better fitting one

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

As mentioned by @weaverryan in #3706 since Nginx is supported the name ``$APACHEUSER`` doesn't any more match its usage really properly. Thus, I renamed it to ``$HTTPDUSER``.