Add magic command for VCS SSH authenticity keys
Currently, `checkout` does some magic, including adding authenticity keys for Bitbucket and GitHub SSH connections. While steps are being taken to document this, it would be handy to break out that functionality (specifically the adding of the "tried and true" SSH authenticity keys) into a dedicated command, which would allow jobs that need to interact with GitHub/Bitbucket an easy way to add these keys without it being too verbose in the configuration. Jobs that implement `checkout` silently benefit from this, but it's not clear that this is the case.
Something like `add_vcs_keys` or something?