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
tests/unit: refactor reading neutron.conf.test

neutron.conf.test includes rpc_backend whose value is stashed.
Thus it is required to reset when tearing down, otherwise the stale status
will be used by succeeding tests causing random error.

This patch refactors reading neutron.conf.test and resets the status of
rpc_backend properly.

Closes-Bug: #1281481
Closes-Bug: #1284549
Change-Id: I0fa5945b6adbb9945d353028ec88d00ccbf4e31a
Merge "Allow ML2 plugin test cases to be run independently"
Merge "tests/unit: refactor reading neutron.conf.test"