[COOK-3372] unzip webpi if webpi executable doesn't exist

Original pull request from HH here:

    https://github.com/opscode-cookbooks/webpi/pull/4

The not_if guard protects us from unzipping this on every run and
removing the `action :nothing` ensure that the file will get unzip on
the next run even if the previous run failed before it could be
unzipped but after it was downloaded.