Fix domain initialization

Created by: NotSpecial

Eve does not allow schemas to contain fields such as _id or _etag.

However, we want to create them for better documentation.

This is no problem if we add them after Eve's check (which cannot be disabled :/), but in the current implementation, this would all mutate the domain dict defined in settings.

Thus, if a second app were to be created in the same python interpreter, the domain would already be filled and Eve would complain.

Until now, this was only an issue with the tests, so I reset the domain after each test run. However, due to a small mistake, the CLI would call create_app twice.

Normally, this should not be an issue. However, the situation explained above would cause Eve to complain and crash.

Now I decided to fix the problem more properly:

Instead of a fixed dict in settings.py, a fresh dict is now initialized in create_app.