provider/azurerm: Fixing a bug in azurerm_network_interface

[tombuildsstuff]

When using this Terraform configuration, which creates multiple NIC's in different Subnets within the same Virtual Network using either Terraform 0.9.4 / the master branch this returns an error:

Error applying plan:

2 error(s) occurred:

* azurerm_network_interface.test2: 1 error(s) occurred:

* azurerm_network_interface.test2: network.InterfacesClient#CreateOrUpdate: Failure responding to request: StatusCode=429 -- Original Error: autorest/azure: Service returned an error. Status=429 Code="RetryableError" Message="A retryable error occured." Details=[{"code":"ReferencedResourceNotProvisioned","message":"Cannot proceed with operation because resource /subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tharvey-dev7986/providers/Microsoft.Network/virtualNetworks/tharveydev7986nw/subnets/tharveydev7986sn1 used by resource /subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tharvey-dev7986/providers/Microsoft.Network/networkInterfaces/tharveydev7986ni2 is not in Succeeded state. Resource is in Updating state and the last operation that updated/is updating the resource is PutSubnetOperation."}]
* azurerm_network_interface.test1: 1 error(s) occurred:

* azurerm_network_interface.test1: network.InterfacesClient#CreateOrUpdate: Failure responding to request: StatusCode=429 -- Original Error: autorest/azure: Service returned an error. Status=429 Code="RetryableError" Message="A retryable error occured." Details=[{"code":"ReferencedResourceNotProvisioned","message":"Cannot proceed with operation because resource /subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tharvey-dev7986/providers/Microsoft.Network/virtualNetworks/tharveydev7986nw/subnets/tharveydev7986sn1 used by resource /subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tharvey-dev7986/providers/Microsoft.Network/networkInterfaces/tharveydev7986ni1 is not in Succeeded state. Resource is in Updating state and the last operation that updated/is updating the resource is PutSubnetOperation."}]

When provisioning using the changes in this PR, which ensures we only modify one subnet/virtual network at a time - this gets provisioned as expected:

Apply complete! Resources: 11 added, 0 changed, 0 destroyed.

Fixes #7986

[tombuildsstuff]

Ran the entire test suite - the tests pass (ignoring some unrelated/intermittent failures):

[paddycarver]

I had a minor nitpick, but this looks fine to me. Let's wait for @stack72 to weigh in, though, before merging this.

[paddycarver]

Why return a pointer to a slice? A slice is basically a pointer already, so I don't really see the benefit.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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