Move ohai build definition from omnibus-software to omnibus-chef

`ohai` software definition contains custom logic (appbundler integration) for Chef omnibus packages. Move it to the omnibus-chef repo so that custom omnibus packages don't break based on upstream changes.

Recommended way to install ohai should be from Rubygems.

**This should be backported to master as well.``

/cc: @opscode/client-engineers, @schisamo