##### 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

vm_dict['properties']['networkProfile']['networkInterfaces'] = updated_nics
                        changed = True
                if self.os_disk_caching and \
                   self.os_disk_caching != vm_dict['properties']['storageProfile']['osDisk']['caching']:
                    self.log('CHANGED: virtual machine {0} - OS disk caching'.format(self.name))
                    differences.append('OS Disk caching')
                    changed = True
                    vm_dict['properties']['storageProfile']['osDisk']['caching'] = self.os_disk_caching
                if self.os_disk_name and \
                   self.os_disk_name != vm_dict['properties']['storageProfile']['osDisk']['name']:
                    self.log('CHANGED: virtual machine {0} - OS disk name'.format(self.name))
                    differences.append('OS Disk name')
                    changed = True
                    vm_dict['properties']['storageProfile']['osDisk']['name'] = self.os_disk_name
                if self.os_disk_size_gb and \
                   self.os_disk_size_gb != vm_dict['properties']['storageProfile']['osDisk'].get('diskSizeGB'):
                    self.log('CHANGED: virtual machine {0} - OS disk size '.format(self.name))
                    differences.append('OS Disk size')
                    changed = True
                    vm_dict['properties']['storageProfile']['osDisk']['diskSizeGB'] = self.os_disk_size_gb
                if self.vm_size and \
                   self.vm_size != vm_dict['properties']['hardwareProfile']['vmSize']:
                    self.log('CHANGED: virtual machine {0} - size '.format(self.name))
                    differences.append('VM size')
                    changed = True
                    vm_dict['properties']['hardwareProfile']['vmSize'] = self.vm_size
                update_tags, vm_dict['tags'] = self.update_tags(vm_dict.get('tags', dict()))
                if update_tags:
                    differences.append('Tags')
                    changed = True