Update Azure MSI endpoint

[mbrancato]

Update the MSI endpoint appropriate for either App Service or instance metadata. Fixes Azure storage with MSI on App Service.

Fixes #10623

[kalafut]

This LGTM.

(For others, ref to the new function: adal/token.go)

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3 out of 4 committers have signed the CLA.

• VioletHynes
• mbrancato
• tomcf-hcp
• Jim Kalafut

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[VioletHynes]

Hi there! This looks reasonable enough to me. It seems that both of these have been deprecated since, but the suggested change makes it better, as I understand it. I'll see about getting this merged. Sorry that this waited for so long!

Thanks for the contribution!