Support creating disk resources and attaching them to VMs

[Tarmil]

Currently in Farmer, VM data disks are always specified in the VM resource and created with createOption = "Empty". ARM also supports creating disks as a separate resource, and attaching them to a VM with createOption = "Attach". Something like this in Farmer would be nice:

let myDisk = disk {
    name "myvm-db-disk"
    size 1024
    disk_type Standard_LRS
}

let myVm = vm {
    name "myvm"
    attach_disk myDisk
}