debian: Transition to dh_python2

debian: Transition to dh_python2

dh_pysupport that the packaging used until now is deprecated, with
dh_python2 as its successor.

This commit removes the PYTHONPATH setting from
debian/openvswitch-ipsec.init because it is not needed, as the Python
packaging is public.  In fact, the Python packaging was public,
unintentionally, before, so the PYTHONPATH could have been removed earlier.