Consul Docs: Azure auto-join MSI clarification

[nphilbrook]

Description

While figuring out how to use Azure auto-join from VMs with both a System-managed identity and a User-managed identity, when I needed to use the User-managed identity, I stumbled on some serious pain. This is probably not the best way to explain it, but I wanted others to benefit from my journey of pain. I will happily accept any suggestions on wording, organization, or verbiage. The 2 main points I want to get across that I discovered from reading both the go-discover code and the Azure/go-autorest code are that:

• client_id, client_secret, and tenant_id must ALL be specified for any of them to be respected. If you omit any of them, none of them are respected and MSI is used. Code reference from go-discover
• Once you fall back to MSI, to specify a specific client_id, you have to use AZURE_CLIENT_ID and NOT ARM_CLIENT_ID. Code references from Azure/go-autorest:
  • https://github.com/Azure/go-autorest/blob/c2958ac74c65c138045daa60f149c5f9d949d743/autorest/azure/auth/auth.go#L43
  • https://github.com/Azure/go-autorest/blob/c2958ac74c65c138045daa60f149c5f9d949d743/autorest/azure/auth/auth.go#L198

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[boruszak]

Please implement suggestions for style guide alignment.

[boruszak]

LGTM!