[Snap] Implement Wi-Fi hotspot on first boot

[benfrancis]

As a user I want to connect to the gateway as a Wi-Fi hotspot when it first boots, so that I can connect the gateway to a network of my choosing.

Currently with Raspbian-based images when the gateway first boots it acts as a Wi-Fi hotspot with a captive portal which a user can connect to from another device (e.g. smartphone/tablet/laptop) in order to connect the gateway to a Wi-Fi network.

This will need implementing separately for the snap package when bundled with a custom Ubuntu Core OS image. This could be part of the gateway snap, or possibly a separate one.

For inspiration see:

• WIP hotspot - closes #59 krellian/kiosk#79
• https:/ogra1/opennds-snap
• https:/ogra1/nm-wifi-hotspot-snap
• https://git.launchpad.net/~snappy-hwe-team/snappy-hwe-snaps/+git/wifi-connect/tree/
• https://git.launchpad.net/~snappy-hwe-team/snappy-hwe-snaps/+git/wifi-ap/tree/bin/ap.sh?h=stable