Support for Ohai hints

This patch adds support for writing Ohai hints to the machine during bootstrap. The knife bootstrap code does this in https://github.com/opscode/chef/blob/master/lib/chef/knife/bootstrap/chef-full.erb

`knife ec2` and friends can be used to set hints but we also add a --hint option to `knife solo prepare` in order to use it with instances that have already been created.

The code is currently run directly from bootstrap! which might not be ideal. Is there a better place to put it?