OS API: support for multiple versions in an OS

Allow multiple api versions in an OS. This is according to the OS API
changes design doc, by which an OS can support multiple versions of the
Ganeti API and if one is supported by Ganeti it will work. Since up to
version 5 of the API mandates an OS could support only one version, this
change is retrocompatible with it and requires no version bump up.

Reviewed-by: iustinp