Ensure $HOME is set to `node[:rbenv][:user]` before git clones

This is a workaround for CHEF-3940. TL;DR Certain versions of `git` misbehave if configuration is inaccessible in $HOME. We work around this issue by ensuring $HOME is temporarily set to the `node[:rbenv][:user]` user before performing a `git` operation.

This PR also adds Test Kitchen support and some initial Bats integration tests.