Support re-issue certificates; Updated CF to use SAM; Upgrade to nodejs8 /w promises

[brightsparc]

Issue #6 and #9:

Description of changes:

• Enhancement to support re-issuing a certificate when exists in Dynamo DB (schema unchanged)
• Refactored index.js to use SAM API gateway with lambda proxy (updated CF template).
• Refactored app.js to return promise for all methods to simplify callbacks/exception management.
• Added support for getting desired and updating reporting IoT shadow.

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

[t04glovern]

Fantastic job, code works really well! Excited to see this moved into master