##### 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

boot_diagnostics=self.compute_models.BootDiagnostics(
                                enabled=self.boot_diagnostics['enabled'],
                                storage_uri=boot_diag_storage_account.primary_endpoints.blob))
                    if self.admin_password:
                        vm_resource.os_profile.admin_password = self.admin_password
                    if self.custom_data:
                        
                        vm_resource.os_profile.custom_data = to_native(base64.b64encode(to_bytes(self.custom_data)))
                    if self.os_type == 'Linux' and os_profile:
                        vm_resource.os_profile.linux_configuration = self.compute_models.LinuxConfiguration(
                            disable_password_authentication=disable_ssh_password
                        )
                    if self.ssh_public_keys:
                        ssh_config = self.compute_models.SshConfiguration()
                        ssh_config.public_keys = \
                            [self.compute_models.SshPublicKey(path=key['path'], key_data=key['key_data']) for key in self.ssh_public_keys]
                        vm_resource.os_profile.linux_configuration.ssh = ssh_config
                    
                    if self.data_disks:
                        data_disks = []
                        count = 0
                        for data_disk in self.data_disks:
                            if not data_disk.get('managed_disk_type'):
                                if not data_disk.get('storage_blob_name'):
                                    data_disk['storage_blob_name'] = self.name + '-data-' + str(count) + '.vhd'
                                    count += 1
                                if data_disk.get('storage_account_name'):
                                    data_disk_storage_account = self.get_storage_account(data_disk['storage_account_name'])
                                else:
                                    data_disk_storage_account = self.create_default_storage_account()