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 #300 from dirkmueller/master

Disable checksum offloading by default
Only add ETHTOOL_OPTIONS when offloading is disabled

Otherwise use the defaults. This is to workaround
https://bugzilla.suse.com/show_bug.cgi?id=900112
Which causes wickedd to fail to bring up virtio interfaces on SLES12 when tx/rx
options are present in ETHTOOL_OPTIONS.
Merge pull request #305 from rhafer/nic-offload

Only add ETHTOOL_OPTIONS when offloading is disabled
Merge pull request #301 from rhafer/cleanup

smaller code cleanup
Disable rx checksumming by default (bnc#910298)