Add rspec tests

This set of changes adds some rspec-puppet tests
- The outline comes from https://github.com/garethr/puppet-module-skeleton which includes beaker tests, but at this point I've stripped those out to simplify getting this accepted
- I had to update gentoo/portage dependency to 2.1.0 and make a few other minor fixes to get things working at all, based on real testing on CentOS
- puppet-lint is set to allow warnings; I had to make a few minor changes to fix the lint errors. I'll do more de-linting later, but didn't want to include that in this initial commit since it will touch nearly every file in the repository

This is the first set of rspec or rspec-puppet tests I've written outside of exercises, so if there's a better way to structure the tests I'd be happy to take a different approach. Coverage is currently relatively low but the most basic use cases are covered. Gentoo is currently not tested for at all but should be simple enough to add in.