##### 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

rand = random.randrange(1000, 9999)
            storage_account_name = storage_account_name_base + str(rand)
            if self.check_storage_account_name(storage_account_name):
                valid_name = True
                break
        if not valid_name:
            self.fail("Failed to create a unique storage account name for {0}. Try using a different VM name."
                      .format(self.name))
        try:
            account = self.storage_client.storage_accounts.get_properties(self.resource_group, storage_account_name)
        except CloudError:
            pass
        if account:
            self.log("Storage account {0} found.".format(storage_account_name))
            self.check_provisioning_state(account)
            return account
        sku = self.storage_models.Sku(name=self.storage_models.SkuName.standard_lrs)
        sku.tier = self.storage_models.SkuTier.standard
        kind = self.storage_models.Kind.storage
        parameters = self.storage_models.StorageAccountCreateParameters(sku=sku, kind=kind, location=self.location)
        self.log("Creating storage account {0} in location {1}".format(storage_account_name, self.location))
        self.results['actions'].append("Created storage account {0}".format(storage_account_name))
        try:
            poller = self.storage_client.storage_accounts.create(self.resource_group, storage_account_name, parameters)
            self.get_poller_result(poller)
        except Exception as exc:
            self.fail("Failed to create storage account: {0} - {1}".format(storage_account_name, str(exc)))
        self.tags['_own_sa_'] = storage_account_name
        return self.get_storage_account(storage_account_name)
    def check_storage_account_name(self, name):