sbus: dectect python binary for sbus_generate.sh
    
    We already detect python2 and python3 binaries during configure. With
    this patch PYTHON_EXEC is set to the python3 binary if python3 bindings
    are generated and to the python2 binary otherwise. With the help of an
    environment variable sbus_generate.sh is made aware of it.
    
    Related to https://pagure.io/SSSD/sssd/issue/3807
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>