provider/azurerm: avoid registering providers already registered, add option to disable

[pmcatominey]

Fixes #10931.
Fixes #8824.

The provider now consults the Providers API to detect which providers are already
registered and uses this call to test the credentials

A new option skip_provider_registration / ARM_SKIP_PROVIDER_REGISTRATION is
now available to opt out of provider registration entirely.

The error message from the call used to test credentials has also been changed, this will give us better errors once Azure/go-autorest#101 makes it's way into the SDK. Example of current errors:

# Bad Client ID
* Unable to list provider registration status, it is possible that this is due to invalid credentials or the service principal does not have permission to use the Resource Manager API, Azure error: autorest#WithErrorUnlessStatusCode: POST https://login.microsoftonline.com/00000000-0000-0000-0000-000000000000/oauth2/token?api-version=1.0 failed with 400 Bad Request: StatusCode=400

# Bad Client Secret
* Unable to list provider registration status, it is possible that this is due to invalid credentials or the service principal does not have permission to use the Resource Manager API, Azure error: autorest#WithErrorUnlessStatusCode: POST https://login.microsoftonline.com/00000000-0000-0000-0000-000000000000/oauth2/token?api-version=1.0 failed with 401 Unauthorized: StatusCode=401

# Bad Subscription ID
* Unable to list provider registration status, it is possible that this is due to invalid credentials or the service principal does not have permission to use the Resource Manager API, Azure error: resources.ProvidersClient#List: Failure responding to request: StatusCode=404 -- Original Error: autorest/azure: Service returned an error. Status=404 Code="SubscriptionNotFound" Message="The subscription '00000000-0000-0000-0000-000000000000' could not be found."

# Bad Tenant ID
* Unable to list provider registration status, it is possible that this is due to invalid credentials or the service principal does not have permission to use the Resource Manager API, Azure error: autorest#WithErrorUnlessStatusCode: POST https://login.microsoftonline.com/00000000-0000-0000-0000-000000000000/oauth2/token?api-version=1.0 failed with 400 Bad Request: StatusCode=400

[jen20]

This looks like a good idea to me. 👍 for moving to the official SDK where practical - the current solution was always a hack.

[jen20]

Thanks @pmcatominey! I've resolved the conflicts and merged.

[ghost]

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