Add ability to set variables stored on the server

[joesolly]

Add an api for server.save() so things like environment variables and api keys can be saved without being hard-coded into the code.

[cat-haines]

The Build API doesn't allow you to set environmental variables/server.save variables meaning we'd need to push a new firmware version to set the environmental variables.

This means that setting environmental variables would cause the device/agent to restart - is that an acceptable side effect? I suppose it's probably better than nothing at all?