Enable appbundler for chef

1. Enable appbundler for chef
2. Move ohai software definition to omnibus-chef since it contains appbundler specific logic (recommended pattern for using ohai in custom omnibus packages should be to install it from Rubygems).

Accompanying PRs: https://github.com/opscode/ohai/pull/407, https://github.com/opscode/omnibus-software/pull/290

My plan is to port these on top of @schisamo's omnibus 4 branch to minimize the conflicts. This change is for Chef 12 and Chef DK (therefore **master** branch) only.

/cc: @opscode/client-engineers 
this is smelling like a new omnibus DSL method