Enhance the AppConfig with external config sources. (#1904)

* Enhance the AppConfig with external config sources.

The external config sources are currently environment variables
and AWS secrets manager.

The config file can be augmented with a section describing how
environmen variables and secrets can update config parameters.

benefits:
 - it will enable the config to draw from more than one secret.  This is useful
   for shared secrets between cellxgene and data portal, as well as auth0 secrets.
 - it will make it very straightforward to check the config before a deployment.

 Part of #1859

server/common/aws_secret_utils.py

@@ -18,6 +18,6 @@ def get_secret_key(region_name, secret_name):
             return secret
     except Exception as e:
         logging.critical(f"Caught exception during get_secret_key, {e}", exc_info=True)
-        raise SecretKeyRetrievalError
+        raise SecretKeyRetrievalError(str(e))
 
     return None