IoTHub: support for the Basic SKU

[tombuildsstuff]

The Basic SKU is now available in version 2018-04-01 of the IoTHub API - this PR updates to this version of the SDK and adds the property

cc @echuvyrov

[tombuildsstuff]

Tests pass:

$ acctests azurerm TestAccAzureRMIotHub_

=== RUN   TestAccAzureRMIotHub_importBasic
--- PASS: TestAccAzureRMIotHub_importBasic (139.64s)
=== RUN   TestAccAzureRMIotHub_importStandard
--- PASS: TestAccAzureRMIotHub_importStandard (241.72s)
=== RUN   TestAccAzureRMIotHub_basic
--- PASS: TestAccAzureRMIotHub_basic (197.14s)
=== RUN   TestAccAzureRMIotHub_standard
--- PASS: TestAccAzureRMIotHub_standard (125.13s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	703.986s

[tombuildsstuff]

hey @echuvyrov

Just to let you know that we've just shipped support for this as a part of v1.13.0 of the AzureRM Provider :)

Thanks!

[ghost]

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