late initialize fields that get defaulted by AWS

[michaelhtm]

Issue #2695 && #2622

Description of changes:
Some spec fields get default values from AWS after they are created.
These defaults cause the controller to keep requeuing, trying to set
them to null. With these changes, we late initialize these fields,
ensuring they are defaulted if the user does not define them

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knottnt, michaelhtm

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[ack-prow]

@michaelhtm: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
rds-verify-code-gen	5f602da	link	false	/test rds-verify-code-gen

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