Change Signer to use the config provider and fix tests

Fixes the broken ACI tests

endpoints/web.py

@@ -105,7 +105,7 @@ def aci_signing_key():
   if not signer.name:
     abort(404)
 
-  return send_file(signer.public_key_path)
+  return send_file(signer.open_public_key_file())
 
 @web.route('/plans/')
 @no_cache