Authentication: Refreshing the Service Principal Token before using it

[tombuildsstuff]

This fix was originally contributed by @sophos-jeff in #1349 but has been split-out

This allows Azure CLI auth to be used to accessing Key Vaults, which fixes #656.

[tombuildsstuff]

Tests run fine for a Service Principal:

$ acctests azurerm TestAccAzureRMResourceGr
=== RUN   TestAccAzureRMResourceGroup_importBasic
--- PASS: TestAccAzureRMResourceGroup_importBasic (69.23s)
=== RUN   TestAccAzureRMResourceGroup_basic
--- PASS: TestAccAzureRMResourceGroup_basic (61.88s)
=== RUN   TestAccAzureRMResourceGroup_disappears
--- PASS: TestAccAzureRMResourceGroup_disappears (71.84s)
=== RUN   TestAccAzureRMResourceGroup_withTags
--- PASS: TestAccAzureRMResourceGroup_withTags (76.84s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	279.853s

and testing this for CLI auth works as expected too 👍

[monkey-jeff]

Nice! Glad to see this bug fixed. Since it'll be a bit before I get back to that PR (I hope I can get to it next week)

[radeksimko]

I don't really have the Azure knowledge here, but the solution looks reasonable to me.

I assume the auth logic in general would be difficult to test.

[tombuildsstuff]

I assume the auth logic in general would be difficult to test.

yeah, it seems fine in my testing so 👍

[ghost]

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