[Snap] Implement SSH Settings

[benfrancis]

In the web interface in Settings -> Developer there is an "Enable SSH" checkbox, which on other platforms enables a user to turn SSH access on and off.

E.g. see https://github.com/WebThingsIO/gateway/blob/master/src/platforms/linux-raspbian.ts#L463

[benfrancis]

This is a bit more complicated for Ubuntu Core. Whereas on Raspbian there is a default username and password but SSH is disabled by default, on Ubuntu Core it's the opposite. There is no default username and password but the user must associate the device with an Ubuntu One account in order to log in via SSH using an SSH key.

I'm not exactly sure what the SSH setting should do in this case. Perhaps the checkbox should be checked but disabled unless the user has manually created a local username and password after logging in with an SSH key. But is that even possible?

[ogra1]

Well, there is actually a setting to en/disable ssh in UbuntuCore ... see https://snapcraft.io/docs/system-options#heading--ssh but that would indeed require the snapd-control interface which we're unlikely to get access to for the snap in the global store ... Perhaps simply showing some explanation/tooltip in the UI for the global version how it is handled on core and what the user should/can do with an additional pointer to the "pro" version from a brand store that allows using this feature would be possible ?