Fix Traceback when running neutron-ipset-cleanup tool

Some of the tools dependencies expect the AGENT group to be
present in the configuration, and at present it is not initialized,
this patch addresses that.

Change-Id: I1a50e77749aaecc3966c9d238f91a1968ed454ef
Signed-off-by: Frode Nordahl <frode.nordahl@canonical.com>
Closes-Bug: #1881771
(cherry picked from commit d57735ae0f)

neutron/cmd/ipset_cleanup.py

@@ -19,6 +19,7 @@ from oslo_log import log as logging
 from neutron.agent.linux import utils
 from neutron.common import config
 from neutron.conf.agent import cmd as command
+from neutron.conf.agent import common as agent_config
 
 
 LOG = logging.getLogger(__name__)
@@ -31,6 +32,7 @@ def setup_conf():
     from the main config that do not apply during clean-up.
     """
     conf = cfg.CONF
+    agent_config.register_root_helper(conf=conf)
     command.register_cmd_opts(command.ip_opts, conf)
     return conf