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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 -->

else:
                        
                        
                        param = [x for x in disk.keys() if x.startswith('size_')][0]
                        unit = param.split('_')[-1]
                        disk_size = disk[param]
                        if isinstance(disk_size, (float, int)):
                            disk_size = str(disk_size)
                        try:
                            if re.match(r'\d+\.\d+', disk_size):
                                
                                expected = float(disk_size)
                            else:
                                
                                expected = int(disk_size)
                        except (TypeError, ValueError, NameError):
                            disk_size_parse_failed = True
                    if disk_size_parse_failed:
                        
                        self.module.fail_json(msg="Failed to parse disk size for disk index [%s],"
                                                  " please review value provided"
                                                  " using documentation." % disk_index)
                    disk_units = dict(tb=3, gb=2, mb=1, kb=0)
                    unit = unit.lower()
                    if unit in disk_units:
                        current_disk['size'] = expected * (1024 ** disk_units[unit])
                    else:
                        self.module.fail_json(msg="%s is not a supported unit for disk size for disk index [%s]."
                                                  " Supported units are ['%s']." % (unit,
                                                                                    disk_index,