CLI should support roles and MFA

[jpignata]

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See this post for implementation details.

[jpignata]

This may take a little doing to add as the golang SDK doesn't cache credentials meaning a user would have to provide his or her MFA code for each command. We'd want to add a cache first so that users would only have to supply the MFA code once every 8 or so hours.

[osterman]

Is this really necessary? We're using aws-vault to achieve the same result. Fwiw, it's in go, so if you need a reference implementation - there you have it! =)