Add OLSRd gateway selection support

[areynold]

We already use SmartGateway, so it should be possible to create an interface to allow users to select their gateway based on certain criteria. This would ideally integrate with service advertisements and servald signing, so that we would have more criteria upon which to select than simple ETX.

(Was issue 364 on code.commotionwireless.net)

[seamustuohy]

Per: opentechinstitute/luci-commotion#85 @jheretic
I will add UCI bindings to add options and values to the /etc/config/network "plug" network. Please let me know what you would like the final options and values named and I will update them in the interface. If we should combine some of these let me know.

1. for advertising the gateway over the mesh
   option advertised (true/false)
2. for configuring the way dhcp is handled on the interface
   option configuration (client/host/auto)
3. for turning on meshing on the interface
   option meshed (true/false)

[jheretic]

@elationfoundation Sounds good. I would maybe change the second option to be 'dhcp' rather than 'configuration.' However, this issue is for allowing a node to select a gateway from a list other than the one that OLSRd considers to be the default, not the option that we discussed yesterday. That one is #43

[seamustuohy]

Darn... woops! I will use those values then.

[jheretic]

@elationfoundation Not to hijack this issue further, but there is one further option: client_bridge, which holds the configuration name of the bridge which the plug interface now tries to add itself to when it fails to obtain a lease.