Use python_pip resource instead of execute

There was a lot of duplication across recipes using the `execute` resource to do a source install for each component, whisper, carbon, and graphite-web. This is part of gradual changes toward a more a library style cookbook.

This switches to using the `python_pip` resource provided by the python cookbook. The result is much cleaner recipes for whisper, carbon and web.

Source installs are still possible if desired by changing the new `node['graphite']['install_type']` attribute, which will use the url attributes for each component as defined in the `package_name` attribute hash. This also makes use of lazy attributes to allow flexible setting of the value as needed.

Converges on centos 6 and ubuntu 12.04, though other issues affect passing tests.