support cross account ELB Alias

[yteraoka]

I want to add alias record that pointed to ELB name owned by other AWS account.
AWS API require hosted_zone_id, so specify it in Routefile like following.

  rrset "elb.example.com.", "A" do
    dns_name "some-other-accounts-elb.ap-northeast-1.elb.amazonaws.com.", :hosted_zone_id => 'ABCDEFGHIJKLMN'
  end

[winebarrel]

Thanks!