Adds performance insights to Postgres template

[O5m4r]

• yamllint state/rds-postgres.yaml
• cfn-lint -i E1019 E3002 E2520 -t state/rds-postgres.yaml
• aws cloudformation validate-template --template-body file://state/rds-postgres.yaml

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Added parameters to optionally have performance insights in Postgres RDS, and custom amount of retention days in it.

[michaelwittig]

@andreaswittig could you check the defaults? Is it a good idea to enable by default or will this break existing setups with small instances? Not sure if we should get rid of PerformanceInsights and turn off if PerformanceInsightsRetentionPeriod is set to zero.

[andreaswittig]

@O5m4r Thanks a lot for your contribution.

Your solution is very flexible but comes with the disadvantage, that setting PerformanceInsight to true will fail for some instance types.

I took a different approach for the wordpress/wordpress-ha template. My idea was to enable RDS Performance Insights for all supported instance types out of the box. See #490. What do you think?

[O5m4r]

I like the Idea of having a mapping, to make sure we don't allow Performance Insights in an Instance that doesn't support it.