We need to refactor all common ground for custom ways to exec.Command() and friends

[avnik]

Now we have few own wrappers on exec.Command() inside resource/lvm/lowlevel, resource/shell, and resource/user. Would be nice to refactor it to common, easy mockable thin wrapper, with well defined behaviour, disticnction on non-zero return code from program from error.

Also would be nice to have likewise injection points to read/write fliles, and various os.* stuff like os.Getuid()