Allow ML2 plugin test cases to be run independently

Allow ML2 plugin test cases to be run independently

Some test cases in neutron.tests.unit.ml2.test_ml2_plugin override the
network_vlan_ranges option in the ml2_type_vlan config group. However,
the test module does not import the ML2 type driver module where this
config group's options are registered. When unit tests are run by
tox/testr the module is imported during test discovery. Running the
tests independently via nose or testtools requires the option to be
explicitly imported.

Change-Id: I355a2d1212992eafed44f6d843c0fc9deb665df7
Closes-bug: #1287792