VMSS: ensuring we always set the vhd_containers field

[tombuildsstuff]

Issue #1043 shows a Diff Mismatch where the vhd_containers field isn't set; this PR fixes that by ensuring the field is always set.

Fixes #1043

[mbfrahry]

Do you mind throwing a test on this? I'd just like to see that extra bit of confirmation that the issue is fixed

[tombuildsstuff]

Looking into adding a test for this - since this is a set with a dynamic computed value (since the storage account name is being used, which has to be globally unique) - I don't think there's an easy way of computing this:

  storage_profile_os_disk.1237888334.vhd_containers.#:                                          "" => "1"
  storage_profile_os_disk.1237888334.vhd_containers.1346120568:                                 "" => "https://tomdevvmsssa21.blob.core.windows.net/vhds"
  tags.%:                                                                                       "" => "<computed>"

As such I'm going to suggest we merge this without a test specifically for this, since the Import tests are covering it?

[bflad]

Nested sets are a pain for testing, but I've personally used similar fixes. The import testing should cover this at least from the "no worse" standpoint. LGTM if the existing acceptance testing is happy. 👍

[tombuildsstuff]

Tests pass:

[ghost]

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