package/python-rtslib-fb: update URL to the open-iscsi organization

package/python-rtslib-fb: update URL to the open-iscsi organization

Also add a comment to indicate that python-configshell-fb and
targetcli-fb should be bumped every time python-rtslib-fb is bumped
because upstream usually upgrades all of these packages at the same
time.

Another comment warns that switching this package to PyPI would make us
miss new versions of python-rtslib-fb.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>