refactoring appinsights use the new sdk version

[mbfrahry]

Migrating appinsights to the new sdk version

make testacc TEST=./azurerm TESTARGS="-run=TestAccAzureRMApplicationInsights"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./azurerm -v -run=TestAccAzureRMApplicationInsights -timeout 180m
=== RUN   TestAccAzureRMApplicationInsights_importBasicWeb
--- PASS: TestAccAzureRMApplicationInsights_importBasicWeb (58.32s)
=== RUN   TestAccAzureRMApplicationInsights_importBasicOther
--- PASS: TestAccAzureRMApplicationInsights_importBasicOther (41.72s)
=== RUN   TestAccAzureRMApplicationInsights_basicWeb
--- PASS: TestAccAzureRMApplicationInsights_basicWeb (40.62s)
=== RUN   TestAccAzureRMApplicationInsights_basicOther
--- PASS: TestAccAzureRMApplicationInsights_basicOther (55.58s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	196.283s

[tombuildsstuff]

one minor nit about the imports, but this otherwise LGTM 👍

[tombuildsstuff]

Given the package insights is used in multiple places within Azure (e.g. #478), I think it might be sensible to alias this import - I'll push a commit to update this

[ghost]

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