Better Call Home OS detection

We currently do a great job of detecting an OS pmm-managed runs on in general case, but that general case has too little in common with our case. 

If we run pmm-managed in OVF, OS should not be detected at all, has to be a fixed value "OVF".

If we run pmm-managed in Docker container, then we should find a way to detect a host OS. Seems the best thing we can do is to parse /proc/version (note the "16.04.4" in the output).

The end result: we get more accurate telemetry results for Linux distribution and version.