[COOK-3615] Required UFW package is not installed on Debian

https://tickets.opscode.com/browse/COOK-3615

Debian (unlike Ubuntu) does not install the UFW package by default.

The LWRPs in the cookbook appear to work fine once it has been installed. I've made the package install as part of the default recipe, updated the documentation to say that it should be added to the run list as well as adding some tests to ensure we can successfully converge on Debian and Ubuntu.