Dhcp timeout

This should be tested in conjunction with the dhcp-timeout pull request in luci-commotion: https://github.com/opentechinstitute/luci-commotion/pull/10

To test:
* start up node when connected to a gateway, and ensure it gets a DHCP lease.
* set dhcp_timeout to 1 in /etc/config/commotiond, reboot, and make sure it didn't get a lease this time
* remove the dhcp_timeout option from /etc/config/commotiond, reboot, and it should still get a DHCP lease (20 seconds is the default timeout)