##### 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

requested_vhd_uri = None
        data_disk_requested_vhd_uri = None
        disable_ssh_password = None
        vm_dict = None
        image_reference = None
        custom_image = False
        resource_group = self.get_resource_group(self.resource_group)
        if not self.location:
            
            self.location = resource_group.location
        self.location = normalize_location_name(self.location)
        if self.state == 'present':
            
            if self.vm_size and not self.vm_size_is_valid():
                self.fail("Parameter error: vm_size {0} is not valid for your subscription and location.".format(
                    self.vm_size
                ))
            if self.network_interface_names:
                for nic_name in self.network_interface_names:
                    nic = self.parse_network_interface(nic_name)
                    network_interfaces.append(nic)
            if self.ssh_public_keys:
                msg = "Parameter error: expecting ssh_public_keys to be a list of type dict where " \
                    "each dict contains keys: path, key_data."
                for key in self.ssh_public_keys:
                    if not isinstance(key, dict):
                        self.fail(msg)
                    if not key.get('path') or not key.get('key_data'):
                        self.fail(msg)
            if self.image and isinstance(self.image, dict):