Use `host` as default value for `authKey`

This way, one can just use the hostname on .ftppass, like so:

Gruntfile.js

    ftp-deploy': {
      build: {
        auth: { host: 'somehost.com', port: 21 }
    }}

.ftppass

    {
      "somehost.com": {
        "username": "someusername",
        "password": "somepassword"
    }}