Fix OS_VARIANT variable setting

This was introduced in efaa9b06.

 in OSCoreEnv:
  inst_os.name is pure operating system name (without variant) as variant is stripped
   in OSFromDisk(). So we always get variant = inst_os.supported_variants[0] (first
   variant in variants list).
  Adding argument os_name with full name (including variant) solves this problem.

Signed-off-by: Vitaly Kuznetsov <vitty@altlinux.ru>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
[modified by iustin to handle the call to OSCoreEnv from ValidateOS too]
Reviewed-by: Michael Hanselmann <hansmi@google.com>