Support more platforms

This change aims to make things work more smoothly on most platforms, but there are a few gotchas remaining:

* `nrsysmond` suite passes on all platforms
* `plugin-agent` suite is broken on fedora until something like https://github.com/opscode-cookbooks/python/pull/26 can be merged in the `python` cookbook
* `generic-agent` suite is broken on centos and fedora until something like https://github.com/jtimberman/ruby-cookbook/pull/3 can be merged in the `ruby` cookbook

Given the state of Fedora support in other cookbooks we depend on, and what I consider the low probability that someone would want to install New Relic tools on a Fedora system in a production environment, maybe we should reconsider supporting Fedora as a target platform?