Allow `/dev` devices pass-trough to container

We need sometimes to have ability to pass some /dev/* device to container (for example /dev/ppp). The problem is that this might require some kernel module that might not be available so we should check for its existence and possibly skip test with some mark.

• implement /dev existence mark (to skip tests if device is not available)
• implement LXD container device to pass arbitrary /dev/* to container This is blocker for https://gitlab.labs.nic.cz/turris/nsfarm/issues/10