##### 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

self.fail("parameter error: plan must include name, product, and publisher")
            if not self.storage_blob_name and not self.managed_disk_type:
                self.storage_blob_name = self.name + '.vhd'
            elif self.managed_disk_type:
                self.storage_blob_name = self.name
            if self.storage_account_name and not self.managed_disk_type:
                properties = self.get_storage_account(self.storage_account_name)
                requested_storage_uri = properties.primary_endpoints.blob
                requested_vhd_uri = '{0}{1}/{2}'.format(requested_storage_uri,
                                                        self.storage_container_name,
                                                        self.storage_blob_name)
            disable_ssh_password = not self.ssh_password_enabled
        try:
            self.log("Fetching virtual machine {0}".format(self.name))
            vm = self.compute_client.virtual_machines.get(self.resource_group, self.name, expand='instanceview')
            self.check_provisioning_state(vm, self.state)
            vm_dict = self.serialize_vm(vm)
            if self.state == 'present':
                differences = []
                current_nics = []
                results = vm_dict
                
                if self.network_interface_names:
                    for nic in vm_dict['properties']['networkProfile']['networkInterfaces']:
                        current_nics.append(nic['id'])
                    if set(current_nics) != set(network_interfaces):
                        self.log('CHANGED: virtual machine {0} - network interfaces are different.'.format(self.name))
                        differences.append('Network Interfaces')
                        updated_nics = [dict(id=id, primary=(i == 0))
                                        for i, id in enumerate(network_interfaces)]