Persistent, unique `FIREZONE_ID` across deploys

[jamilbk]

Google's instance group manager doesn't provide an easy way to use replica index or some other deterministic source of data to uniquely identify Firezone Gateways deployed by this module.

So we use uuidgen to generate a random one per instance on provision. This doesn't cause an issue per se, it just creates more Gateways in the admin portal than needed.

This could be solved if we could pass a random base string + deterministic replica index as the FIREZONE_ID variable.