Functional test for ipv6 subnet can handle multiple allocation pools

- also added import of plugin.py since quotas won't be imported without it if test run individually
    - eample `tox -e py27 -- tests/funcation/plugin_modules/test_subnets.py` will fail without the import line
    - add #noqa so flake8 doesn't complain

RM9834
Just for this test file, yes.  plugin.py gets loaded up through levels of __init__.py inclusions when you run the whole suite.