Fix RDS storage quota "Units set to None" error

[bosmanx]

Summary

Fix "Units set to "None"" error when retrieving RDS "Total storage for all DB instances" quota from Service Quotas. AWS changed the quota unit from "Gigabytes" to "None" without announcement, similar to the ELB issue fixed in #503. This PR adds a unit converter that accepts both "Gigabytes" and "None" units.

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