Add option for client to authenticate with IAM token server.

[smithsz]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

Add option for client to authenticate with IAM token server.

Fixes #419.

Approach

Add additional kwargs iam_client_id and iam_client_secret to Cloudant client constructor.

Schema & API Changes

Adds additional kwargs. No breaking API changes.

Security and Privacy

No change.

Testing

Includes additional mock unit test.

Monitoring and Logging

No change.