(gh-456) Can't ssh root@127.0.0.1 via vagrant if root...

... login disabled in /etc/ssh/sshd_config

- add enable_root_login step by default
- clean up spurious 'end' when generating/using an extra disk on
  virtualbox instances
- tested clean locally
Merge pull request #517 from anodelman/vagrant-maint

(gh-456) Can't ssh root@127.0.0.1 via vagrant if root...