Create a constant for waiting times

[javigomez]

after this comment #33 (comment), Yves suggested to create a constant that will be used for all waiting times.

Constants in codeception can be defined in different places. One possible place could be the _bootstrap file: https://github.com/joomla-projects/gsoc16_browser-automated-tests/blob/staging/tests/acceptance/_bootstrap.php

[javigomez]

Please @pritalpatel send a new pull request updating the comments at #47 (comment)