Fix import of public ip of a network interface

[thiagocaiubi]

Fix runtime error on conversion of *string to string when presence of public ip

Before:

$ make testacc TEST=./azurerm TESTARGS="-run
TestAccAzureRMNetworkInterface_importPublicIP"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./azurerm -v -run
TestAccAzureRMNetworkInterface_importPublicIP -timeout 120m
=== RUN   TestAccAzureRMNetworkInterface_importPublicIP
panic: interface conversion: interface {} is *string, not string

goroutine 393 [running]:
github.com/terraform-providers/terraform-provider-azurerm/azurerm.resourceArmNetworkInterfaceIpConfigurationHash(0x183ad00, 0xc4201aa4b0, 0xc4202a9478)
        /Users/thiagocaiubi/Development/go/src/github.com/terraform-providers/terraform-provider-azurerm/azurerm/resource_arm_network_interface_card.go:401 +0xa75
github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema.(*Set).hash(0xc4202a9460, 0x183ad00, 0xc4201aa4b0, 0x10, 0x20)
        /Users/thiagocaiubi/Development/go/src/github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema/set.go:180 +0x3d
github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema.(*Set).add(0xc4202a9460, 0x183ad00, 0xc4201aa4b0, 0x1884d00, 0x1, 0xc4202a9460)
        /Users/thiagocaiubi/Development/go/src/github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema/set.go:167 +0x81
github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema.(*Set).Add(0xc4202a9460, 0x183ad00, 0xc4201aa4b0)
        /Users/thiagocaiubi/Development/go/src/github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema/set.go:69 +0x44
github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema.NewSet(0x198c5e8, 0xc4202fac30, 0x1, 0x1, 0xc4202fac00)
        /Users/thiagocaiubi/Development/go/src/github.com/terraform-providers/terraform-provider-azurerm/vendor/github.com/hashicorp/terraform/helper/schema/set.go:56 +0xa2
github.com/terraform-providers/terraform-provider-azurerm/azurerm.resourceArmNetworkInterfaceRead(0xc4201cda40, 0x1875c20, 0xc4202ee000, 0xc4203690e0, 0x0)

After:

$ make testacc TEST=./azurerm TESTARGS="-run
TestAccAzureRMNetworkInterface_importPublicIP"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./azurerm -v -run
TestAccAzureRMNetworkInterface_importPublicIP -timeout 120m
=== RUN   TestAccAzureRMNetworkInterface_importPublicIP
--- PASS: TestAccAzureRMNetworkInterface_importPublicIP (120.69s)
PASS
ok github.com/terraform-providers/terraform-provider-azurerm/azurerm 120.801s

[tombuildsstuff]

Hey @thiagocaiubi

Thanks for this PR - I've taken a look and this LGTM. The tests pass:

$ TF_ACC=1 envchain azurerm go test ./azurerm -v -timeout 300m -parallel 5 -run TestAccAzureRMNetworkInterface_

=== RUN   TestAccAzureRMNetworkInterface_importBasic
--- PASS: TestAccAzureRMNetworkInterface_importBasic (122.32s)
=== RUN   TestAccAzureRMNetworkInterface_importIPForwarding
--- PASS: TestAccAzureRMNetworkInterface_importIPForwarding (104.43s)
=== RUN   TestAccAzureRMNetworkInterface_importWithTags
--- PASS: TestAccAzureRMNetworkInterface_importWithTags (103.86s)
=== RUN   TestAccAzureRMNetworkInterface_importMultipleLoadBalancers
--- PASS: TestAccAzureRMNetworkInterface_importMultipleLoadBalancers (161.74s)
=== RUN   TestAccAzureRMNetworkInterface_importPublicIP
--- PASS: TestAccAzureRMNetworkInterface_importPublicIP (123.31s)
=== RUN   TestAccAzureRMNetworkInterface_basic
--- PASS: TestAccAzureRMNetworkInterface_basic (117.47s)
=== RUN   TestAccAzureRMNetworkInterface_disappears
--- PASS: TestAccAzureRMNetworkInterface_disappears (168.43s)
=== RUN   TestAccAzureRMNetworkInterface_enableIPForwarding
--- PASS: TestAccAzureRMNetworkInterface_enableIPForwarding (117.67s)
=== RUN   TestAccAzureRMNetworkInterface_multipleLoadBalancers
--- PASS: TestAccAzureRMNetworkInterface_multipleLoadBalancers (141.21s)
=== RUN   TestAccAzureRMNetworkInterface_withTags
--- PASS: TestAccAzureRMNetworkInterface_withTags (116.26s)
=== RUN   TestAccAzureRMNetworkInterface_bug7986
--- PASS: TestAccAzureRMNetworkInterface_bug7986 (119.54s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	1396.245s

Thanks!

[ghost]

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