OSEnvironment: populate OS_VARIANT

According to the design on api_version >= 15 the OS variant is the part
of the OS name after the "+" sign. If none is found, we just pass in the
first variant an OS declares (which is bound to exist, as we check for
it in _TryOSFromDisk).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>