Add verify to nipap-passwd

Add new feature to nipap-passwd to verify if a username and password is correct. The tool will show if the user exists and if the password is correct.

For request https://github.com/SpriteLink/NIPAP/issues/467
I'm fine with having a "user_exists" function, but if you are going to use it to fetch a username, it shouldn't be named used_exists but rather "get_user". I would expect "user_exists" to return True / False, while if it actually returns a user, it should be called get_user or similar.

For the purpose of checking whether a user exists, I think it would be fine to do something along the lines of
```
if not self.get_user(username):
    # user not found, bla bla bla
```
ie, get_user would return None (or raise an exception!) if the user can't be found.