Documentation/target: add an example script to configure an iSCSI target

The script illustrates how to interact with configfs to create a very
simple LIO iSCSI target with a file or block device backstore.

The script can serve as a starting point for people that cannot use
targetcli because Python is not available on their machine.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: Vincent Donnefort <vdonnefort@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>