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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 -->

"""
        Check correctness of disk input provided by user
        Returns: A list of dictionary containing disk information
        """
        disks_data = list()
        if not self.desired_disks:
            self.module.exit_json(changed=False, msg="No disks provided for virtual"
                                                     " machine '%s' for management." % self.vm.name)
        for disk_index, disk in enumerate(self.desired_disks):
            
            current_disk = dict(disk_index=disk_index,
                                state='present',
                                datastore=None,
                                autoselect_datastore=True,
                                disk_unit_number=0,
                                scsi_controller=0)
            
            if 'state' in disk:
                if disk['state'] not in ['absent', 'present']:
                    self.module.fail_json(msg="Invalid state provided '%s' for disk index [%s]."
                                              " State can be either - 'absent', 'present'" % (disk['state'],