add prefer_database_credentials configuration parameter

When true, users' Google Cloud Storage credentials will be written to
the data provider instead of disk.
Pre-existing credentials on disk will be used as a fallback

Fixes #201

config/config.go

@@ -141,7 +141,8 @@ func init() {
 					Parallelism: 2,
 				},
 			},
-			UpdateMode: 0,
+			UpdateMode:                0,
+			PreferDatabaseCredentials: false,
 		},
 		HTTPDConfig: httpd.Conf{
 			BindPort:           8080,