Allow storage update operation to unavailable db instance

[michaelhtm]

Issue #2214

Description of changes:
With this change we allow the controller to make a storage update
call when the DB instance is unavailable due to a storage-full status.

This case is not easy to test, since the minimum storage to allocate is 20GiB.
Options for testing:

• Create an existing DB instance that is already unavailable due to storage-full
  status and update storage (this would then require us to fill up the remaining space)

more ideas are welcome...

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

[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-attribution	ab5af3a	link	false	/test rds-verify-attribution

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[a-hilaly]

🚀 🚀
/lgtm

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: a-hilaly, michaelhtm

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