Fixes#7299 where it was found that computer_name is not optional (as
the msdn documentation states)
```
make testacc TEST=./builtin/providers/azurerm TESTARGS='-run=TestAccAzureRMVirtualMachine_'
==> 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 (403.53s)
=== RUN TestAccAzureRMVirtualMachine_tags
--- PASS: TestAccAzureRMVirtualMachine_tags (488.46s)
=== RUN TestAccAzureRMVirtualMachine_updateMachineSize
--- PASS: TestAccAzureRMVirtualMachine_updateMachineSize (601.82s)
=== RUN TestAccAzureRMVirtualMachine_basicWindowsMachine
--- PASS: TestAccAzureRMVirtualMachine_basicWindowsMachine (646.75s)
=== RUN TestAccAzureRMVirtualMachine_windowsUnattendedConfig
--- PASS: TestAccAzureRMVirtualMachine_windowsUnattendedConfig (891.42s)
=== RUN TestAccAzureRMVirtualMachine_winRMConfig
--- PASS: TestAccAzureRMVirtualMachine_winRMConfig (768.73s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/azurerm 3800.734s
```
af4cc20ec0