Add Azure auth method

[Ramesh7]

This PR is about adding the Azure Auth method as described here :
https://www.vaultproject.io/docs/auth/azure#via-the-api

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[ssethi19]

Hi,

Can you provide the status of this PR?

[dhuckins]

@Ramesh7 can you sign the CLA (see bot comment for details) and pull down the latest? we just made some CI changes

thanks!