provider/azurerm: VM Scale Sets - import support + fixes

[tombuildsstuff]

Fixes several issues with VM Scale Sets:

• Adds import support
• Fixing a bug where a null value was appended to the ssh_keys state field, causing a crash on the next apply, which fixes: make apply crashes #13094 azure_virtual_mashine_scale_set: crash after apply on created environment #13009
• Storing custom_data in the state file as base64
• Making custom_data ForceNew, since it's not possible to update it

[grubernaut]

LGTM

[grubernaut]

LGTM!

[tombuildsstuff]

Tests pass:

$ envchain azurerm make testacc TEST=./builtin/providers/azurerm/ TESTARGS='-run=TestAccAzureRMVirtualMachineScaleSet'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/12 12:39:48 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/azurerm/ -v -run=TestAccAzureRMVirtualMachineScaleSet -timeout 120m
=== RUN   TestAccAzureRMVirtualMachineScaleSet_importBasic
--- PASS: TestAccAzureRMVirtualMachineScaleSet_importBasic (564.11s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_importLoadBalancer
--- PASS: TestAccAzureRMVirtualMachineScaleSet_importLoadBalancer (568.10s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_importOverProvision
--- PASS: TestAccAzureRMVirtualMachineScaleSet_importOverProvision (639.96s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_importExtension
--- PASS: TestAccAzureRMVirtualMachineScaleSet_importExtension (532.32s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_importMultipleExtensions
--- PASS: TestAccAzureRMVirtualMachineScaleSet_importMultipleExtensions (652.82s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_basic
--- PASS: TestAccAzureRMVirtualMachineScaleSet_basic (647.19s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_linuxUpdated
--- PASS: TestAccAzureRMVirtualMachineScaleSet_linuxUpdated (657.71s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_basicLinux_disappears
--- PASS: TestAccAzureRMVirtualMachineScaleSet_basicLinux_disappears (440.78s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_loadBalancer
--- PASS: TestAccAzureRMVirtualMachineScaleSet_loadBalancer (575.16s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_overprovision
--- PASS: TestAccAzureRMVirtualMachineScaleSet_overprovision (1139.42s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_extension
--- PASS: TestAccAzureRMVirtualMachineScaleSet_extension (648.13s)
=== RUN   TestAccAzureRMVirtualMachineScaleSet_multipleExtensions
--- PASS: TestAccAzureRMVirtualMachineScaleSet_multipleExtensions (668.92s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/azurerm

[grubernaut]

[ghost]

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