Be more explicit about code that's only needed on Ubuntu

This should make the code better understandable. And as and additional benefit
save us some CPU/IO cycles during the chef-client run.
Adjust resource- and file names to match reality

Nowadays we explicitly enable netfilter on bridges for all nodes. The name of
the chef resource and the configfile on disk however where still suggesting it
was being disabled. Also try to clean up the old misnamed file if it is still
present.
Merge pull request #301 from rhafer/cleanup

smaller code cleanup