Fix option parsing for destination arg

As mentionned at http://docs.python.org/3.4/library/argparse.html#nargs,
nargs=1 will not provide the default behaviour, but instead will produce
a list of args, and that is not what we want.

This patch uses nargs=None to provide default behaviour instead.

As it is a positionnal argument, we can't use required option.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>