Force host dns resolution for guest

Issue on osx 10.9.2 with virtualbox 4.3.8 where dns queries through certain programs like curl were timing out. Error was thrown in vagrant up for bootstrap.saltstack.org . 

Adding in config line from :https://leap.se/code/issues/1089 resolved the issue.
Yeah, this is going to break a lot of stuff. Can you revert this section @pdewouters ? Thanks!