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
Default Slave#environment to nil instead of {}
Cannot call super on DSL methods
Suppress output from the update_center JSON
Version bump v2.1.1
Allow users to be created with already hashed passwords.