Have a sane way managing omnibus.rb files in client pipelines

Before this is merged: 

* Something similar to below needs to be deployed in chefdk slaves @ https://github.com/opscode-cookbooks/opscode-ci/blob/master/libraries/omnibus_build_pipeline.rb#L199-L200

```
  case platform
  when "windows"
    cp omnibus.rb.#{project.name}.windows omnibus.rb
  when "mac"
    cp omnibus.rb.#{project.name}.mac omnibus.rb
  else
    cp omnibus.rb.#{project.name}.linux omnibus.rb
  end
```

* Gemfile should be updated once https://github.com/opscode/omnibus-ruby/pull/128 is taken care of.

/cc: @opscode/client-eng , @opscode/release-engineers