Add test framework for executing from within Foreman

To use this, load the plugin into Foreman as normal using
a "gem ..." line in Bundler. Then in `rake -T` you will see
a new task `test:default_hostgroup` when can be run in isolation
and should also be called from `rake test`
Clean up unnecessary files
Fixes #2 - Don't blow up with an invalid Hostgroup setting