Proxmox-builder pass through boot-order

Adding the boot field supported by Proxmox api and proxmox-api-go library.
This is due to a change in Proxmox to allow for multiple boot harddrives which would break the packer proxmox builder (the hardcoded cdn boot value).

Can be specified like the following example:     
      "disks": [
        {
          "type": "scsi",
          "disk_size": "20G",
          "storage_pool": "vmstorage",
          "storage_pool_type": "lvm"
        }
      ],
      "boot": "order=scsi0;ide2",
      "iso_file": "media:iso/{{ user `ubuntu_iso_file` }}"

Not sure this is ideal, but it's a workaround for the #10252 issue.

builder/proxmox/common/config.go

@@ -39,6 +39,7 @@ type Config struct {
 	VMName string `mapstructure:"vm_name"`
 	VMID   int    `mapstructure:"vm_id"`
 
+	Boot           string       `mapstructure:"boot"`
 	Memory         int          `mapstructure:"memory"`
 	Cores          int          `mapstructure:"cores"`
 	CPUType        string       `mapstructure:"cpu_type"`

builder/proxmox/common/step_start_vm.go

@@ -43,7 +43,7 @@ func (s *stepStartVM) Run(ctx context.Context, state multistep.StateBag) multist
 		Name:         c.VMName,
 		Agent:        agent,
 		QemuKVM:      kvm,
-		Boot:         "cdn", // Boot priority, c:CDROM -> d:Disk -> n:Network
+		Boot:         c.Boot, // Boot priority, example: "order=virtio0;ide2;net0", virtio0:Disk0 -> ide0:CDROM -> net0:Network
 		QemuCpu:      c.CPUType,
 		Description:  "Packer ephemeral build VM",
 		Memory:       c.Memory,