Use the update-center when installing plugins

This commit tells Chef to download and install the update-center when
installing plugins. This will permit the installation of plugin
dependencies, much like when plugins are installed via the GUI.
Handle super calls correctly in load_current_resource
Backport Chef patches into the cookbook
Bump metadata to 2.1.1
Bump metadata again to 2.1.2 (previous version with temporary hacks was 2.1.1, rebased upstream to get rid of hacks)