By default, the `vagrant rsync` command assumes that the SSH user
belongs to a group of the same name [1]:
# Folder options
opts[:owner] ||= ssh_info[:username]
opts[:group] ||= ssh_info[:username]
Although consumers can override this behavior, Vagrant Boxes which
follow this convention will provide a more seamless experience. Document
the convention in the instructions for Vagrant Box authors.
[1] 3d68e16f1b/plugins/synced_folders/rsync/helper.rb (L73-L75)
