##### 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

vm_resource.license_type = self.license_type
                    if self.boot_diagnostics is not None:
                        vm_resource.diagnostics_profile = self.compute_models.DiagnosticsProfile(
                            boot_diagnostics=self.compute_models.BootDiagnostics(
                                enabled=vm_dict['properties']['diagnosticsProfile']['bootDiagnostics']['enabled'],
                                storage_uri=vm_dict['properties']['diagnosticsProfile']['bootDiagnostics']['storageUri']))
                    if vm_dict.get('tags'):
                        vm_resource.tags = vm_dict['tags']
                    
                    if vm_dict['properties'].get('osProfile', {}).get('customData'):
                        custom_data = vm_dict['properties']['osProfile']['customData']
                        
                        vm_resource.os_profile.custom_data = to_native(base64.b64encode(to_bytes(custom_data)))
                    
                    if vm_dict['properties'].get('osProfile', {}).get('adminPassword'):
                        vm_resource.os_profile.admin_password = vm_dict['properties']['osProfile']['adminPassword']
                    
                    linux_config = vm_dict['properties'].get('osProfile', {}).get('linuxConfiguration')
                    if linux_config:
                        ssh_config = linux_config.get('ssh', None)
                        vm_resource.os_profile.linux_configuration = self.compute_models.LinuxConfiguration(
                            disable_password_authentication=linux_config.get('disablePasswordAuthentication', False)
                        )
                        if ssh_config:
                            public_keys = ssh_config.get('publicKeys')
                            if public_keys:
                                vm_resource.os_profile.linux_configuration.ssh = self.compute_models.SshConfiguration(public_keys=[])
                                for key in public_keys:
                                    vm_resource.os_profile.linux_configuration.ssh.public_keys.append(
                                        self.compute_models.SshPublicKey(path=key['path'], key_data=key['keyData'])