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##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
vmware_guest_disk should propose `disk_mode` parameter (like vmware_guest module).
The playbook bellow does not raise an error but disk is always added in `persistent` mode, even I set `independent_persistent` for `disk_mode` var.
If it's a bug please could you fix it? If it's not consider it as a feature request :)

##### ISSUE TYPE
- Feature Idea (or maybe a bug)

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
vmware_guest_disk 

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml
---
- name: get vm {{ target | upper }} facts
  vmware_guest_facts:
    hostname: "{{ vcenter_hostname }}"
    username: "{{ vcenter_username }}"
    password: "{{ vcenter_password }}"
    datacenter: "{{ datacenter }}"
    validate_certs: no
    name: "{{ target | upper }}"
  register: vm_facts
  delegate_to: localhost

- name: add disk to vm {{ target | upper }}
  vmware_guest_disk:
    hostname: "{{ vcenter_hostname }}"
    username: "{{ vcenter_username }}"
    password: "{{ vcenter_password }}"
    datacenter: "{{ datacenter }}"
    validate_certs: no
    name: "{{ target | upper }}"
    disk:
      - size_gb: "{{ size_gb }}"
        type: "{{ type }}"
        disk_mode: "{{ disk_mode }}"
        scsi_controller: "{{ scsi_controller }}" 
        unit_number: "{{ unit_number }}"
        datastore: "{{ vm_facts.instance.hw_datastores | first }}"
  delegate_to: localhost
```

<!--- HINT: You can also paste gist.github.com links for larger files -->

for disk in disk_data:
            disk_change = False
            scsi_controller = disk['scsi_controller'] + 1000  
            if disk['disk_unit_number'] not in current_scsi_info[scsi_controller]['disks'] and disk['state'] == 'present':
                
                disk_spec = self.create_scsi_disk(scsi_controller, disk['disk_unit_number'])
                disk_spec.device.capacityInKB = disk['size']
                if disk['disk_type'] == 'thin':
                    disk_spec.device.backing.thinProvisioned = True
                elif disk['disk_type'] == 'eagerzeroedthick':
                    disk_spec.device.backing.eagerlyScrub = True
                disk_spec.device.backing.fileName = "[%s] %s/%s_%s_%s.vmdk" % (disk['datastore'].name,
                                                                               vm_name, vm_name,
                                                                               str(scsi_controller),
                                                                               str(disk['disk_unit_number']))
                disk_spec.device.backing.datastore = disk['datastore']
                self.config_spec.deviceChange.append(disk_spec)
                disk_change = True
                current_scsi_info[scsi_controller]['disks'][disk['disk_unit_number']] = disk_spec.device
                results['disk_changes'][disk['disk_index']] = "Disk created."
            elif disk['disk_unit_number'] in current_scsi_info[scsi_controller]['disks']:
                if disk['state'] == 'present':
                    disk_spec = vim.vm.device.VirtualDeviceSpec()
                    
                    disk_spec.device = current_scsi_info[scsi_controller]['disks'][disk['disk_unit_number']]
                    
                    if disk['size'] < disk_spec.device.capacityInKB:
                        self.module.fail_json(msg="Given disk size at disk index [%s] is smaller than found (%d < %d)."
                                                  " Reducing disks is not allowed." % (disk['disk_index'],
                                                                                       disk['size'],