rename OSMajor.osversion and OSMajor.osminor properties to OSMajor.osversions

Up until 0.16 we had both OSMajor.osversion and OSMajor.osminor meaning
the same thing (list of OSVersion). During the conversion to SQLAlchemy
declarative in 0.16, OSMajor.osversion was accidentally dropped.

This patch renames them both to OSMajor.osversions, which more
accurately reflects what the property really is.

Bug: 1078941
Change-Id: I90497858c2487ad578fec4673dae8e7b15871a93