Cleanup trailing white spaces
Disable checksum offloading by default

Most 1g/10g drivers seem to have issues with checksum offloading
when vlans are in use. As those issues are hard to debug and easy
to workaround, lets disable checksum offloading by default.

Add a hidden option in the network.json to be able to set it
to enabled.
Merge pull request #300 from dirkmueller/master

Disable checksum offloading by default