digitalocean create doesn't support --ssh-key and --ssh-pub-key arguments

[andrewchilds]

If --ssh-key is specified when running overcast digitalocean create, it will use overcast.key during creation, but after that any connection attempts will use the specified key pair, which won't work. overcast digitalocean create should be consistent with other provider commands and support --ssh-key and --ssh-pub-key arguments.