use packagecloud cookbook

This provides RHEL/CentOS compatibility.

Tested on CentOS 6.5, will do 7 soon

add provider from packagecloud to manage chef/stable repo
add packagecloud cookbook dependency
We can remove the `case` block and use `value_for_platform_family` in the packagecloud_repo resource, I believe.

```ruby
packagecloud_repo 'chef/stable' do
  type value_for_platform_family(:debian => 'deb', :rhel => 'rpm')
end
```