Have a sane way of managing omnibus.rb files in the pipelines.
Pick up windows-compat-defaults of omnibus-ruby.
Make client pipelines build dmgs.

Since `Omnibus.config[:sign_pkg] = false` is not working because omnibus is loading all the project files, temporarily switching over to having an additional file for client pipelines until we enable signing on chefdk slaves.
Update libyaml to 0.1.6 on windows.
Testing disk-cleanup branch of omnibus-ruby.