Changed default wired interface names to 'wan'

This addresses #122, and should be used in concert with the equivalent pull request opentechinstitute/luci-commotion#205.

To test:
1. After flashing without retaining settings, make sure you can connect to the node via ethernet.
2. Run through setup_wizard. After it completes, make sure that the ethernet interface works as expected (gets a lease and/or gives out leases automatically).
3. Modify the settings under Additional Network Interfaces, and make sure that the settings that it's applying are used on the 'wan' config section and not 'wired.'