fix: add IAM dummy store for gateway operations (#12670)

with console addition users cannot login with
root credentials without etcd persistent layer,
allow a dummy store such that such functionalities
can be supported when running as non-persistent
manner, this enables all calls and operations.

cmd/iam.go

@@ -452,7 +452,11 @@ func (sys *IAMSys) InitStore(objAPI ObjectLayer) {
 	defer sys.Unlock()
 
 	if globalEtcdClient == nil {
-		sys.store = newIAMObjectStore(objAPI)
+		if globalIsGateway {
+			sys.store = &iamDummyStore{}
+		} else {
+			sys.store = newIAMObjectStore(objAPI)
+		}
 	} else {
 		sys.store = newIAMEtcdStore()
 	}