Fixes #8006 - added STDOUT proxy log option

Although we have it documented in our example file, this was never working:

```
```

Although one can set this to `/dev/stdout`, this does not work on foreman
discovery image under systemd environment because stdout cannot be reopened
there (Permission error). I would like have the possibility to setup STDOUT
this way, because I want to capture the output into systemd journal in the
discovery image environment.

By default, systemd captures stdout/stderr and adds entries to the journal
under the foreman-proxy tag. Therefore on Fedoras and RHEL7+ we can perhaps
check "proper syslog support" for foreman-proxy as well.