Allow overriding user in Hosts

[theRemix]

Current behavior:

• default user is root
• user can be overridden in [Overrides]
• User = "ubuntu" would override every ssh connection to connect as user ubuntu

Desired behavior:

• each host can override the user it connects as
• if there is no inline override, fall back to previous behavior

example recipe:

[Overrides]
  User = "admin"

[Required]
  Hosts = [
    "localhost:221",
    "root@localhost:222",
    "customuser@localhost:223",
    "localhost:224"
  ]

1st connection will connect as user admin 2nd connection will connect as user root 3rd connection will connect as user customuser 4th connection will connect as user admin