Allow for custom RDS_SUPERUSER_PASSWORDs to be set in config.yml

[scottx611x]

• Just brought up all-files-test stack with a custom RDS password successfully.

Fixes #1990

[codecov-io]

Codecov Report

Merging #1991 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1991   +/-   ##
========================================
  Coverage    44.13%   44.13%           
========================================
  Files          415      415           
  Lines        27570    27570           
  Branches      1322     1322           
========================================
  Hits         12169    12169           
  Misses       15401    15401

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