code: public AccessToken retrieveAccessToken(String userName, String password, Scope... scopes) {
          return getAuthService().retrieveAccessToken(userName, password, scopes);
      }
docs: Provide an {@link AccessToken} for the {@link org.osiam.client.oauth.GrantType}
  RESOURCE_OWNER_PASSWORD_CREDENTIALS.
  
  @param userName the userName of the actual User
  @param password the password of the actual User
  @param scopes   the wanted Scopes of the {@link AccessToken}
  @return an valid {@link AccessToken}
  @throws IllegalStateException if OSIAM's endpoint(s) are not properly configured