Release cleanup should be configurable and default to 5 (currently 6)

[cgriego]

The cleanup action in chef-deploy should be configurable, like capistrano, to keep a configurable number of releases or disable the release cleanup. Also, capistrano's default is to keep the last 5 releases, but chef-deploy is cleaning up releases >= 5 instead of > 5 so it's keeping only 4 releases by default.