allow usage of IMDSv2

[clhuang]

Description of changes: Current implementation panics at string(body[0 : len(body)-1]) if the instance requires IMDSv2, as the metadata endpoint returns status 401 and empty body. In this case, instead fetch the metadata token and use that to actually fetch the metadata.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[himberjack]

What's up with that? We really need this. Thanks