What's the best default username?

[tessereth]

If the user doesn't provide a custom username, we need to pick one. Some possible options are:

1. Use a random username. This works fine for mysql and it possibly the best security wise but postgres has permission issues. Unbinding and rebinding will mean your application can't access it's data anymore.
2. Use the app guid. This means unbinding and rebinding the same application to the database works by default but blue-green does not, and neither does deleting and recreating the app.
3. Use the instance guid. This means all apps would use the same username by default. This is the easiest usability-wise but the worst security-wise.

Currently we use option 2 but we should discuss this to decide the best way.