HUE-2405 [useradmin] Add enhanced user password policy

If enabled, the default password policy requires a password that:
1. Must be at least 8 characters long;
2. Must contain uppercase and lowercase letters;
3. Must contain at least one number;
4. Must contain at least one special character.

This security fix applies in the following use cases:
1. Create the superuser's password on login page.
2. Edit superuser's profile.
3. Create/Edit a user's password.

Add test_user_admin_password_policy in useradmin.tests