MDL-43213 try to simplify behat config and init code

1/ always require 3 behat settings - $CFG->behat_wwwroot, $CFG->behat_dataroot and $CFG->behat_prefix
2/ cleanup init code
3/ do not require $CFG->wwwroot and $CFG->dataroot on test sites
4/ remove switch completely - the most confusing part for me
5/ print out behat_wwwwroot in init script so that you can test the test site manually