Vivid is unsupported now, so remove those tests. Add Yakkety tests for any test that had a xenial test.
The one thing that is questionable here is the usage of 'linux-generic' as the fallback, where previously there was none. The change means that linux-generic kernel would be installed even if there was a useful kernel installed in the image.
Fix ifconfig output parsing to work for yakkety, And remove the code duplication by putting the ifconfig parser into helpers.