Expose ID of an embedded subnet defined in azurerm_virtual_network

[JunyiYi]

• Add test cases to verify ID is actually set
• Add documentation and sample code to explain how to reference the subnet ID
• Expose the subnet ID in azurerm_virtual_network resource

Actually it is hard to reference a specific subnet element in the current implementation because of the hash function. But that is a different story and requires state migration as well. So I just keep it as a small change in this PR.

The PR enables user referencing the subnet ID defined in azurerm_virtual_network resource mentioned in issue #666 .

Fixes #666

[tombuildsstuff]

hey @JunyiYi

Thanks for this PR - if we can fix the documentation issues flagged below this otherwise LGTM 👍

Thanks!

[tombuildsstuff]

Now that the sample's been removed this now LGTM 👍

[tombuildsstuff]

Tests pass (now that #1985 has been temporarily reverted):

[ghost]

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