##### SUMMARY
If you happen to use the __azure_rm_virtualmachine__ module to manage a VM powerstate, if no os_disk_caching attribute is not specified, the module sets *ReadOnly* as default value.

Unfortunately, if the already existing VM has a different type of OS disk cache, e.g. *ReadWrite*, the module attempts to update the VM whereas, as os_disk_caching has not been specified, it is not expected.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
* lib/ansible/modules/cloud/azure/azure_rm_virtualmachine.py

##### ANSIBLE VERSION
```paste below
ansible 2.8.5
  config file = /home/***/ansible/ansible_dev.cfg
  configured module search path = [u'/home/***/ansible/library']
  ansible python module location = /home/***/ansible/.venv/lib/python2.7/site-packages/ansible
  executable location = /home/***/ansible/.venv/bin/ansible
  python version = 2.7.5 (default, Jun 11 2019, 14:33:56) [GCC 4.8.5 20150623 (Red Hat 4.8.5-39)]
```

##### CONFIGURATION
```paste below
DEFAULT_BECOME_METHOD(/home/***/ansible/ansible_dev.cfg) = sudo
DEFAULT_BECOME_USER(/home/***/ansible/ansible_dev.cfg) = root
DEFAULT_GATHER_TIMEOUT(/home/***/ansible/ansible_dev.cfg) = 30
DEFAULT_HOST_LIST(/home/***/ansible/ansible_dev.cfg) = [u'/home/***/ansible/inventory/azure-dev']
DEFAULT_LOG_PATH(/home/***/ansible/ansible_dev.cfg) = /home/***/ansible.log
DEFAULT_MODULE_PATH(/home/***/ansible/ansible_dev.cfg) = [u'/home/***/ansible/library']
DEFAULT_ROLES_PATH(/home/***/ansible/ansible_dev.cfg) = [u'/home/***/ansible/roles']
DEFAULT_VAULT_IDENTITY_LIST(/home/***/ansible/ansible_dev.cfg) = []
HOST_KEY_CHECKING(/home/***/ansible/ansible_dev.cfg) = False
INVENTORY_ENABLED(/home/***/ansible/ansible_dev.cfg) = [u'yaml', u'azure_rm', u'script']
RETRY_FILES_ENABLED(/home/***/ansible/ansible_dev.cfg) = False
```

##### OS / ENVIRONMENT
Controller: RHEL 7.4
Target VM: RHEL 7.4


##### STEPS TO REPRODUCE
1. Create VM with "ReadWrite" as disk cache type
1. Try to power it off:
```yaml
- name: Stop Azure VM
  azure_rm_virtualmachine:
    name: "{{ vm_name }}"
    started: no
    state: present
    resource_group: "***"
    subscription_id: "***"
    tenant: "***"
    client_id: "***"
    secret: "***"
```

##### EXPECTED RESULTS
The VM is stopped and the OS disk caching is not updated.

##### ACTUAL RESULTS
The VM OS Disk caching is updated and the VM is stopped.

When debugging the code, there is a difference appened to the __self.differences__ list:
* OS Disk caching

The workaround right now is to specify the __os_disk_caching__ attribute with the OS disk caching type that matches the one of the VM

self.log("data disk storage account:")
                                    self.log(self.serialize_obj(data_disk_storage_account, 'StorageAccount'), pretty_print=True)
                                if not data_disk.get('storage_container_name'):
                                    data_disk['storage_container_name'] = 'vhds'
                                data_disk_requested_vhd_uri = 'https://{0}.blob.{1}/{2}/{3}'.format(
                                    data_disk_storage_account.name,
                                    self._cloud_environment.suffixes.storage_endpoint,
                                    data_disk['storage_container_name'],
                                    data_disk['storage_blob_name']
                                )
                            if not data_disk.get('managed_disk_type'):
                                data_disk_managed_disk = None
                                disk_name = data_disk['storage_blob_name']
                                data_disk_vhd = self.compute_models.VirtualHardDisk(uri=data_disk_requested_vhd_uri)
                            else:
                                data_disk_vhd = None
                                data_disk_managed_disk = self.compute_models.ManagedDiskParameters(storage_account_type=data_disk['managed_disk_type'])
                                disk_name = self.name + "-datadisk-" + str(count)
                                count += 1
                            data_disk['caching'] = data_disk.get(
                                'caching', 'ReadOnly'
                            )
                            data_disks.append(self.compute_models.DataDisk(
                                lun=data_disk['lun'],
                                name=disk_name,
                                vhd=data_disk_vhd,
                                caching=data_disk['caching'],
                                create_option=self.compute_models.DiskCreateOptionTypes.empty,
                                disk_size_gb=data_disk['disk_size_gb'],
                                managed_disk=data_disk_managed_disk,