Having the --privileged option first prevents this error when running unsupported install on ubuntu 20.04, installed through the installer script.
```
CRITICAL (MainThread) [supervisor.misc.hwmon] Not privileged to run udev monitor!
```
* use separate variable to specify Docker config location
* use templates and always adjust paths
This prepares for further customization of file locations.
* allow to customize prefix and sysconfdir
Follow GNU directory variable names and allow to specify PREFIX (by
default /usr) and sysconfdir (by default /etc). This allows to install
hassio scripts and service to alternative locations.