Allow overriding the phantomenv repository

This allows me to use my local hiera configuration to set a phantomenv repository that has been updated recently e.g. I am using this repo: https://github.com/antonio/phantomenv/tree/download_links

The more correct solution to this would be to update phantomenv but unfortunately it doesn't seem like it will happen soon - there's a 2 month old pull request with no feedback here: https://github.com/wfarr/phantomenv/pull/3