Don't install warden into chef omnibus paths

When trying to create a vagrant box I got stuck at the `setup warden` step.
The problem was that the ruby used was the omnibus ruby from the chef install. 
The solution was installing the gems via rvm under the vagrant user.
But to do this I had to use `su vagrant -l -c` because chef currently does not support [doing a login when invoking a script](https://tickets.opscode.com/browse/CHEF-2288).
An alternative would be using [rvm_shell](https://github.com/fnichol/chef-rvm#-rvm_shell).