tests: fixed NoSuchGroupError in TestNovaSegmentNotifier
tests: fixed NoSuchGroupError in TestNovaSegmentNotifier

The tests are failing when executed separately with -m testtools.run
because they rely on another test class to register ml2 and ml2_vlan
option sections.

TrivialFix

Change-Id: I7a9a5c7c7f62783aceccd9e6ea62aacd9753d0d4

noironetworks/neutron,openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,huntxu/neutron,huntxu/neutron