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<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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 -->

def __init__(self, module):
        super(PyVmomiHelper, self).__init__(module)
        self.desired_disks = self.params['disk']  
        self.vm = None
        self.scsi_device_type = dict(lsilogic=vim.vm.device.VirtualLsiLogicController,
                                     paravirtual=vim.vm.device.ParaVirtualSCSIController,
                                     buslogic=vim.vm.device.VirtualBusLogicController,
                                     lsilogicsas=vim.vm.device.VirtualLsiLogicSASController)
        self.config_spec = vim.vm.ConfigSpec()
        self.config_spec.deviceChange = []
    def create_scsi_controller(self, scsi_type, scsi_bus_number):
        """
        Create SCSI Controller with given SCSI Type and SCSI Bus Number
        Args:
            scsi_type: Type of SCSI
            scsi_bus_number: SCSI Bus number to be assigned
        Returns: Virtual device spec for SCSI Controller
        """
        scsi_ctl = vim.vm.device.VirtualDeviceSpec()
        scsi_ctl.operation = vim.vm.device.VirtualDeviceSpec.Operation.add
        scsi_ctl.device = self.scsi_device_type[scsi_type]()
        scsi_ctl.device.unitNumber = 3
        scsi_ctl.device.busNumber = scsi_bus_number
        scsi_ctl.device.hotAddRemove = True
        scsi_ctl.device.sharedBus = 'noSharing'
        scsi_ctl.device.scsiCtlrUnitNumber = 7
        return scsi_ctl
    @staticmethod
    def create_scsi_disk(scsi_ctl_key, disk_index):