Support for removing hanging ports

Sometimes we have situations when all the routers are deleted
but there are still ports with network:router_gateway device
owner. Such ports can be deleted only after clearing the device
owner information.

update_neutron_port() function is added into functest_utils.py
and taken into use in clean_openstack.py.

Change-Id: I1c9ba2ac499edecec99849b5e895be0248ffd84f
Signed-off-by: Viktor Tikkanen <viktor.tikkanen@nokia.com>