Support usernames in Supfile host entries

[qguv]

Add support for a username + @ prefix in Supfile host entries. E.g.:

networks:
  staging:
    hosts:
      - user@host.tld

This was possible before, but buggy. The following bugs are fixed by this PR:

• adding a username to a Supfile host prevents that host matching any configured Host section in ~/.ssh/config
• ~/.ssh/config usernames override the username set in the Supfile