[Snap] Provide option to run on ports 80/443

[benfrancis]

The default ports for the gateway application to serve an HTTP server on (configured in config/default.js) are 8080 and 4443, but when the current Raspbian-based image is built IP tables are configured to forward port 80 to 8080 and port 43 to 443. This is so the gateway application doesn't need to run as root in order to bind to ports 80 and 443.

In the snap package a simple daemon runs as root anyway, so we could just directly bind to port 80 and 443, but the snap needs to be told to do this.

We could either do this by:

• Changing the default ports when building the snap package, so it takes effect for everyone who installs the snap
• Implement a snap configuration so that a user can set the ports using snap set, and then set a different default using a gadget snap when generating a custom Ubuntu Core OS image