roundsman_working_directory is not cleaned

[tf]

When unpacking cookbooks, new cookbook directories are simply copied over existing ones. This means, when changing cookbook versions, old files may remain on the remote machine. In my case, this lead to a hard to find bug, where stuff from a leftover libraries file shadowed some newly introduced definitions.

As a quick fix I added a capistrano task which purges the roundsman_working_directory before each run. Either this should be the default behavior or we need a more sophisticated way of removing old cookbooks before unpacking cookbook.tar.