use a builder method call to create a new game and make diceroller and result calculation an auxiliar method

[plopezgit]

1. review methods and entities envolved (closely related with this old goal: #26 )
2. creates aux method and use it
3. test it
4. merge it

[plopezgit]

this is now the aspect of the game service createGameBy method:

    public void createGameBy(int playerId) {
        gameRepository.save(getGameEntityFromDTO(getTheDiceRolledBy(playerId)));
    }

[plopezgit]

testing

[plopezgit]

manual test passed