tests: functional: Improve creating users in tests
- Do not require a user password when creating a user. Generate a password if not provided.
- Before creating a user, delete previous user with the same name.
- Delete user: Do not skip if the user doesn't exist. It avoids the situation that some @when tests may silently skip.
- Update openvpn and deluge functional tests assuming passwords were not under the tests.
Tests performed:
- users, openvpn and deluge app functional tests pass.