Simplify bootstrap

*Only attempt to install GHE repos if requested*
This works in the same way as GOVUK_REPOS, with an environment variable. The reason for this is that SMEs using this repo will not have access to these repositories and therefore get a nasty error message.

*Simplify bootstrap-vagrant*
- Simplify how gems are installed
- Make librarian puppet install destructive so that broken cached modules are removed and reinstalled rather than staying broken.