Fix diagnostics storage account reference for existing storage account

[ninjarobot]

This PR closes #939

The changes in this PR are as follows:

• Virtual Machines: Fix reference to an existing storage account in boot diagnostics.

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [ ] Updated the documentation in the docs folder for the affected changes.
• [ ] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Bugfix only, no doc to update. Uses existing unit test.

[ninjarobot]

@Vishal7Sharma thank you for reporting #939. Can you please review this fix? I see the issue was that the [reference] depended on the resource being deployed in the same template, which isn't the case when deploying a VM using an existing storage account for boot diagnostics.

[Vishal7Sharma]

Hi @ninjarobot thanks for looking into this, yes, the above PR solves the issue and generates the correct arm template.

[Vishal7Sharma]

Hi @ninjarobot, a minor issue though, now if we pass the parameter "diagnostics_support" without existing storage account, still it generates the arm expression like below. So, I think we have to handle default storage account scenario differently.

"diagnosticsProfile": {
          "bootDiagnostics": {
            "enabled": true,
            "storageUri": "[reference(resourceId('Microsoft.Storage/storageAccounts', 'abcdstorage'), '2019-06-01').primaryEndpoints.blob]"
          }
        },

[ninjarobot]

Thanks @Vishal7Sharma - I must be missing it - what is wrong with that expression?

[Vishal7Sharma]

Hi @ninjarobot, my bad I thought arm expression has to be different for new and existing storage account. I just tested the above changes with new storage account as well and it worked. So, the above changes are correct for both use cases (existing and new). Thanks

[ninjarobot]

Thanks for verifying on your end as well. I'll go ahead and merge to get the fix out for you.