If the app is making a TLS connection to Redis, the new config setting
REDIS_SSLMODE determines whether CA verification should be performed.
Acceptable values are Python `None` or strings "none", "optional", and
"required".
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.
In local development, the app will fail to start if it does not find the
directory specified by CRL_STORAGE_CONTAINER. This adds a few lines to
safely create that directory on startup and corresponding tests.
