Upgrade to Omnibus 4.0

Before merging we'll ensure the following projects can be built:

* [x] ChefDK - http://manhattan.ci.opscode.us/job/chefdk-trigger-ad_hoc/281/downstreambuildview/
* [x] Chef 12 - http://ci.opscode.us/job/chef-client-build/1768/

This branch requires the following PRs also be merged:
https://github.com/opscode/omnibus/pull/338
https://github.com/opscode/omnibus-software/pull/297

I will update WIP commit 2e4925e after the above are merged.

So for RPM-based platforms signing will be disabled since `ENV['OMNIBUS_RPM_SIGNING_PASSPHRASE']` will be nil. On OS X that is more difficult.