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.