Add db alarms

[rnzsgh]

This PR adds optional support for CloudWatch Alarms for the standard RDS engines as well as Aurora MySQL.

The new template bucket is: awslabs-startup-kit-templates-deploy-v2

Alarms publish to a new SNS topic.

Users can choose between standard (default) and enhanced RDS monitoring.

The launch stacks have been updated.

The test harness was updated to validate the extra conditions.

Standard RDS engines have the following alarms (if enabled):

• CPU
• Low free storage space
• Swap usage
• Read latency
• Write latency

Aurora MySQL has alarms for (if enabled):

• CPU
• Select latency
• Insert latency
• Update latency
• Delete latency

Aurora PostgreSQL is not currently supported.