##### 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

subnets = self.network_client.subnets.list(virtual_network_resource_group, virtual_network_name)
            except CloudError:
                self.fail(no_subnets_msg)
            for subnet in subnets:
                subnet_id = subnet.id
                self.log('subnet id: {0}'.format(subnet_id))
                break
            if not subnet_id:
                self.fail(no_subnets_msg)
        pip = None
        if self.public_ip_allocation_method != 'Disabled':
            self.results['actions'].append('Created default public IP {0}'.format(self.name + '01'))
            sku = self.network_models.PublicIPAddressSku(name="Standard") if self.zones else None
            pip_facts = self.create_default_pip(self.resource_group, self.location, self.name + '01', self.public_ip_allocation_method, sku=sku)
            pip = self.network_models.PublicIPAddress(id=pip_facts.id, location=pip_facts.location, resource_guid=pip_facts.resource_guid, sku=sku)
            self.tags['_own_pip_'] = self.name + '01'
        self.results['actions'].append('Created default security group {0}'.format(self.name + '01'))
        group = self.create_default_securitygroup(self.resource_group, self.location, self.name + '01', self.os_type,
                                                  self.open_ports)
        self.tags['_own_nsg_'] = self.name + '01'
        parameters = self.network_models.NetworkInterface(
            location=self.location,
            ip_configurations=[
                self.network_models.NetworkInterfaceIPConfiguration(
                    private_ip_allocation_method='Dynamic',
                )
            ]
        )
        parameters.ip_configurations[0].subnet = self.network_models.Subnet(id=subnet_id)
        parameters.ip_configurations[0].name = 'default'