[COOK-3674] Fixed issue with root pwd

Found an issue: https://tickets.opscode.com/browse/COOK-3674

This fix is the first step toward fixing that. I also added a new set of
regression tests for a non-default data_dir value.

Note: Currently the mysql cookbook breaks on debian when using something other than the default data_dir. My change will also give someone a way to validate there fix (https://tickets.opscode.com/browse/COOK-3527)