[explorer/machine_type] simplify linux add openbsd
This depends on #875 but could be adapted to be independent of it if that MR is deemed inappropriate / undesirable.
By abstracting away vendor-dependent pattern matching for the linux code, we can re-use that and be reasonably sure about OpenBSD machines being virtualised when we can identify the undelrying virtualisation technology. It remains to be solved how to tell if an OpenBSD machine is physical; in that case previous cdist behaviour ("unknown") remains.
For NetBSD something similar to OpenBSD could be done, with different sysctls: hw.machine and hw.model wary of adding those without testing though, so for NetBSD previous cdist behaviour ("unknown") remains. https://netbsd.gw.com/cgi-bin/man-cgi?sysctl+7.i386+NetBSD-9.0