provider/azurerm: Update azurerm_virtual_machine failed on adminPassword

[stack72]

Fixes #6517

The Azure API never returns the AdminPAssword (as is correct) from the Read API call. Therefore on Create, we do not set the AdminPassword of the vm as part of the state. The Same func is used for Create & Update, therefore when we changed anything on the VM, we were getting the following error:

statusCode:Conflict
serviceRequestId:f498a6c8-6e7a-420f-9788-400f18078921
statusMessage:{"error":{"code":"PropertyChangeNotAllowed","target":"adminPassword","message":"Changing property 'adminPassword' is not allowed."}}

To fix this, we need to exclude the AdminPassword from the Update func if it is empty

Test Results:

make testacc TEST=./builtin/providers/azurerm TESTARGS='-run=TestAccAzureRMVirtualMachine' 2>~/tf.log
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/azurerm -v -run=TestAccAzureRMVirtualMachine -timeout 120m
=== RUN   TestAccAzureRMVirtualMachine_basicLinuxMachine
--- PASS: TestAccAzureRMVirtualMachine_basicLinuxMachine (372.34s)
=== RUN   TestAccAzureRMVirtualMachine_updateMachineSize
--- PASS: TestAccAzureRMVirtualMachine_updateMachineSize (579.54s)
=== RUN   TestAccAzureRMVirtualMachine_basicWindowsMachine
--- PASS: TestAccAzureRMVirtualMachine_basicWindowsMachine (723.42s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/azurerm    1675.313s

[jen20]

LGTM!

[ghost]

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