Generate whitespace using the standard function ljust() that is
available in both Py3 [1] and Py2 [2].

1: https://docs.python.org/3/library/stdtypes.html?highlight=strip#str.ljust
2: https://docs.python.org/2.7/library/string.html#string.ljustReviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NRadostin Stoyanov <rstoyanov1@gmail.com>