[COOK-2885] - Add attribute, exclude node

* Set an attribute (caching_server) to true if the node should be a
  caching server. This is done in the cacher-ng recipe.
* Search for the server based on the above attribute.
* Exclude the node itself from search, else apt may fail to install
  packages.
* Update metadata with attributes.
[COOK-2885] - fix errors in metadata.rb
Merge pull request #47 from opscode-cookbooks/COOK-2885

[COOK-2885] - Add attribute, exclude node