Move config path generation into the service config for reusability (#3978)

* Move config path generation into the service config for reusability

* Make NoCostum Default and fix tests

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* remove function not used anymore

* Make path into an option

* remove database_path function and call it directly

* remove helper functions, use consts

substrate/core/service/test/src/lib.rs

@@ -169,8 +169,9 @@ fn node_config<G, E: Clone> (
 		roles: role,
 		transaction_pool: Default::default(),
 		network: network_config,
-		keystore_path: root.join("key"),
+		keystore_path: Some(root.join("key")),
 		keystore_password: None,
+		config_dir: Some(root.clone()),
 		database: DatabaseConfig::Path {
 			path: root.join("db"),
 			cache_size: None