##### 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.storage_profile.data_disks = data_disks
                    
                    if self.accept_terms is True:
                        if not self.plan or not all([self.plan.get('name'), self.plan.get('product'), self.plan.get('publisher')]):
                            self.fail("parameter error: plan must be specified and include name, product, and publisher")
                        try:
                            plan_name = self.plan.get('name')
                            plan_product = self.plan.get('product')
                            plan_publisher = self.plan.get('publisher')
                            term = self.marketplace_client.marketplace_agreements.get(
                                publisher_id=plan_publisher, offer_id=plan_product, plan_id=plan_name)
                            term.accepted = True
                            self.marketplace_client.marketplace_agreements.create(
                                publisher_id=plan_publisher, offer_id=plan_product, plan_id=plan_name, parameters=term)
                        except Exception as exc:
                            self.fail(("Error accepting terms for virtual machine {0} with plan {1}. " +
                                       "Only service admin/account admin users can purchase images " +
                                       "from the marketplace. - {2}").format(self.name, self.plan, str(exc)))
                    self.log("Create virtual machine with parameters:")
                    self.create_or_update_vm(vm_resource, 'all_autocreated' in self.remove_on_absent)
                elif self.differences and len(self.differences) > 0:
                    
                    self.log("Update virtual machine {0}".format(self.name))
                    self.results['actions'].append('Updated VM {0}'.format(self.name))
                    nics = [self.compute_models.NetworkInterfaceReference(id=interface['id'], primary=(i == 0))
                            for i, interface in enumerate(vm_dict['properties']['networkProfile']['networkInterfaces'])]
                    
                    if not vm_dict['properties']['storageProfile']['osDisk'].get('managedDisk'):
                        managed_disk = None