##### 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

for winrm_listener in self.winrm:
                            winrm_listeners.append(self.compute_models.WinRMListener(
                                protocol=winrm_listener.get('protocol'),
                                certificate_url=winrm_listener.get('certificate_url')
                            ))
                            if winrm_listener.get('source_vault'):
                                if not vm_resource.os_profile.secrets:
                                    vm_resource.os_profile.secrets = list()
                                vm_resource.os_profile.secrets.append(self.compute_models.VaultSecretGroup(
                                    source_vault=self.compute_models.SubResource(
                                        id=winrm_listener.get('source_vault')
                                    ),
                                    vault_certificates=[
                                        self.compute_models.VaultCertificate(
                                            certificate_url=winrm_listener.get('certificate_url'),
                                            certificate_store=winrm_listener.get('certificate_store')
                                        ),
                                    ]
                                ))
                        winrm = self.compute_models.WinRMConfiguration(
                            listeners=winrm_listeners
                        )
                        if not vm_resource.os_profile.windows_configuration:
                            vm_resource.os_profile.windows_configuration = self.compute_models.WindowsConfiguration(
                                win_rm=winrm
                            )
                        elif not vm_resource.os_profile.windows_configuration.win_rm:
                            vm_resource.os_profile.windows_configuration.win_rm = winrm
                    if self.boot_diagnostics_present:
                        vm_resource.diagnostics_profile = self.compute_models.DiagnosticsProfile(