It's really annoying that OBS uses Debian_8.0 and Debian_9.0 (for 8.x
and 9.x) and then Debian_10 for 10.x. Let's work around this by some
conditional vars_file trickery, hopefully once and for all.
Change-Id: I45c4752774f44332b3f9aaf875f49043c03ec5ca