PWX-28082: Add support for AWS Secrets Manager

What this PR does / why we need it:

Refactor the existing aws package to incorporate both KMS and Secrets Manager providers.
- Moved the common code into a utils package.
AWS Secrets Manager:
- Implement Get/Put/Delete APIs.
- Add integration tests for these APIs.

Which issue(s) this PR fixes (optional) Closes #

Special notes for your reviewer: Manually tested by running the integration tests with AWS credentials. Trying to see if we can hook that into travis.

libopenstorage / secrets

PWX-28082: Add support for AWS Secrets Manager #65