Prefer subprocess over os.system

Hello,

The use of `os.system` is deprecated and vulnerable. The use of the module [subprocess](https://docs.python.org/3/library/subprocess.html) should be prefered for flexibility, performance and security reasons.

Please see https://github.com/linuxmuster/linuxmuster-linuxclient7/issues/15, as we discussed it ealier with @CodeCrafter912 