Optionally publish to Artifactory using the Omnibus publisher

If the release script is called with the `—-publish-to-artifactory` option each artifact will be published to Artifactory in addition to the normal Omnitruck publishing tasks.

Example of publishing result in Artifactory:

![artifactory_standalone-ubuntu-1404____repository_browser](https://cloud.githubusercontent.com/assets/6164/4082714/3755af10-2ef4-11e4-89b5-8c9214a881a3.png)

The only required CI change will be this small diff in the `chef-client-release-preprod` job:

```diff
@@ -1,4 +1,6 @@
 cd $BUILD_NUMBER
 
+export PATH=/opt/ruby-2.1.2/bin:$PATH
+
 # super jenky way to do this, but we copy over N copies of the client release script and we just need 1
-`find . -name release.rb | head -n 1` -p chef -v $CHEF_VERSION-$CHEF_PACKAGE_ITERATION -b opscode-omnibus-packages-test -c ~/.s3-package-upload-cfg -m ~/.s3-metadata-upload-cfg -M opscode-omnibus-package-metadata-test
+`find . -name release.rb | head -n 1` -p chef -v $CHEF_VERSION-$CHEF_PACKAGE_ITERATION -b opscode-omnibus-packages-test -c ~/.s3-package-upload-cfg -m ~/.s3-metadata-upload-cfg -M opscode-omnibus-package-metadata-test --publish-to-artifactory --omnibus-config ~/omnibus-publish.rb
```

/cc @opscode/release-engineers @opscode/client-engineers 
Just a note: omnibus brings mixlib-shellout. You can probably remove it