'Sets the given entry number to be the default on every next boot
To set a default only for the next boot, use boot_once() instead.
This module performs the same action as client side boottool.py
set_default() method, but with a better name IMHO.
Note: both --set-default=<kernel> and --set-default-index=<index>
on grubby returns no error when it doesn\'t find the kernel or
index. So this method will, until grubby gets fixed, always return
success.
:param index: entry index number to set as the default.'

def set_default_by_index(self, index):

    return self._run_grubby_get_return([('--set-default-index=%s' % index)])
