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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 ds in datastores:
                        if ds.summary.freeSpace > datastore_freespace:
                            
                            datastore = ds
                            datastore_freespace = ds.summary.freeSpace
                    current_disk['datastore'] = datastore
                if 'datastore' not in disk and 'autoselect_datastore' not in disk:
                    self.module.fail_json(msg="Either 'datastore' or 'autoselect_datastore' is"
                                              " required parameter while creating disk for "
                                              "disk index [%s]." % disk_index)
                if [x for x in disk.keys() if x.startswith('size_') or x == 'size']:
                    
                    disk_size_parse_failed = False
                    if 'size' in disk:
                        size_regex = re.compile(r'(\d+(?:\.\d+)?)([tgmkTGMK][bB])')
                        disk_size_m = size_regex.match(disk['size'])
                        if disk_size_m:
                            expected = disk_size_m.group(1)
                            unit = disk_size_m.group(2)
                        else:
                            disk_size_parse_failed = True
                        try:
                            if re.match(r'\d+\.\d+', expected):
                                
                                expected = float(expected)
                            else:
                                
                                expected = int(expected)
                        except (TypeError, ValueError, NameError):
                            disk_size_parse_failed = True