MDL-41592 behat: Allow alt wwwroot to be set for acceptance testing

To set add $CFG->behat_wwwroot = 'http://alturl'; in your config.php
MDL-41592 behat: Config settings more flexible

Adding the option to just set ->behat_wwwroot to
allow switching to the behat test site.
MDL-41592 general: Also checking the site path