Expose an `ohai` method in the project and software DSLs

With the enhanced Cleanroom, the top-level namespace is now `Object` instead
of Omnibus, meaning the Ohai constant is no long available. This commit adds
an `ohai` DSL method to both Project and Software for exposing Ohai data to
the DSLs.
Fixed typo in README
Update the generator template to use #default_root