Support for RDS reader instance

[dchong-revacomm]

Fixes Issue: [link to JIRA ticket](http://example.com)

Description:

Add support for adding a read only instance to the RDS cluster

Security Impact Analysis Questionnaire

Submitter Checklist

• Is there an impact on Auditing and Logging procedures or capabilities?
• Is there an impact on Authentication procedures or capabilities?
• Is there an impact on Authorization procedures or capabilities?
• Is there an impact on Communication Security procedures or capabilities?
• Is there an impact on Cryptography procedures or capabilities?
• Is there an impact on Sensitive Data procedures or capabilities?
• Is there an impact on any other security-related procedures or capabilities?
• No security impacts identified.

Security Risks Identified - For any applicable items on the "Submitter Checklist," describe the impact of the change and any implemented mitigations.