##### 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("Error: fetching virtual network {0} - {1}".format(self.virtual_network_name, str(exc)))
        else:
            
            no_vnets_msg = "Error: unable to find virtual network in resource group {0}. A virtual network " \
                           "with at least one subnet must exist in order to create a NIC for the virtual " \
                           "machine.".format(virtual_network_resource_group)
            virtual_network_name = None
            try:
                vnets = self.network_client.virtual_networks.list(virtual_network_resource_group)
            except CloudError:
                self.log('cloud error!')
                self.fail(no_vnets_msg)
            for vnet in vnets:
                virtual_network_name = vnet.name
                self.log('vnet name: {0}'.format(vnet.name))
                break
            if not virtual_network_name:
                self.fail(no_vnets_msg)
        if self.subnet_name:
            try:
                subnet = self.network_client.subnets.get(virtual_network_resource_group, virtual_network_name, self.subnet_name)
                subnet_id = subnet.id
            except Exception as exc:
                self.fail("Error: fetching subnet {0} - {1}".format(self.subnet_name, str(exc)))
        else:
            no_subnets_msg = "Error: unable to find a subnet in virtual network {0}. A virtual network " \
                             "with at least one subnet must exist in order to create a NIC for the virtual " \
                             "machine.".format(virtual_network_name)
            subnet_id = None
            try: