etcd: enable optional log file for embedded etcd log output

In this commit we add an extra config for enabling logging to an
external file when using embedded etcd. This can be useful when running
integration tests to see more details about etcd related issues.

kvdb/backend.go

@@ -256,7 +256,7 @@ func GetTestBackend(path, name string) (Backend, func(), error) {
 		}
 		return db, empty, nil
 	} else if TestBackend == EtcdBackendName {
-		etcdConfig, cancel, err := StartEtcdTestBackend(path, 0, 0)
+		etcdConfig, cancel, err := StartEtcdTestBackend(path, 0, 0, "")
 		if err != nil {
 			return nil, empty, err
 		}