The application now checks for an environment variable,
OVERRIDE_CONFIG_DIRECTORY. If that value is set, it loops all the files
in the specified directory and checks if their names correspond to known
configuration settings. The contents of any matching files are read and
set as the new configuration value for that setting. This will allow us
to read mounted Azure Key Vault items as config values.
This also moves the functionality for applying environment variables to
the config into an analogous function.
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