cmd/dex: only expand from env for storages and connectors

Bcrypt'd hashes have "$" characters in them. This means that #667
(accepting actually bcrypted values) combined with #627 (expanding
config with environment variables) broke the example config.

For now, allow storages and connectors to expand their configs from
the environment, but don't do this anywhere else.

examples/config-dev.yaml

@@ -58,7 +58,7 @@ enablePasswordDB: true
 staticPasswords:
 - email: "admin@example.com"
   # bcrypt hash of the string "password"
-  hash: "$2a$10$33EMT0cVYVlPy6WAMCLsceLYjWhuHpbz5yuZxu/GAFj03J9Lytjuy"
+  hash: "$2a$10$2b2cU8CPhOTaGrs1HRQuAueS7JTT5ZHsHSzYiFPm1leZck7Mc8T4W"
   username: "admin"
   userID: "08a8684b-db88-4b73-90a9-3cd1661f5466"