Fix azure_rm_manageddisk caching comparison

[nbr23]

SUMMARY

Fix faulty comparison in is_attach_caching_option_different which returned True when caching was "None", and caused disks to be detached / reattached by mistake.

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

• azure_rm_manageddisk

ADDITIONAL INFORMATION

The following code displays the issue. It runs the same action twice, but the result will show as "changed" for both.
This is because the attach_caching is None, but the comparison in the module is incorrect, leading the module to believe it needs to update the caching parameter.
As a result, the disk gets detached from the VM and reattached, which could have unexpected consequences and possibly lead to issues.

    - name: Create data disk and attach to "testvm" virtualmachine
      azure.azcollection.azure_rm_manageddisk:
        resource_group: "{{ resource_group }}"
        name: 'testvm-data'
        location: eastus
        disk_size_gb: 64
        managed_by: testvm
        storage_account_type: Standard_LRS

    - name: Same action again
      azure.azcollection.azure_rm_manageddisk:
        resource_group: "{{ resource_group }}"
        name: 'testvm-data'
        location: eastus
        disk_size_gb: 64
        managed_by: testvm
        storage_account_type: Standard_LRS

The proposed patch fixes it, and the second run shows as unchanged, respecting indempotency.

Cheers!

[xuzhang3]

LGTM