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 
Also here, do we want to collect a list of appbundler apps throughout the build that get appended to an array and then omnibus is responsible for uniq'ing and installing them all at the end?  Then you could just:

```ruby
appbundler << 'chef'
```

the responsibility of adding chef to be appbundler'd should also probably be here all the time, and that line should get removed from the chef-dk definition now.