Reload users upon AddUser on peers (#6975)

Also migrate ReloadFormat to notification subsystem,
remove GetConfig() we do not use this API anymore

cmd/iam.go

@@ -60,9 +60,9 @@ type IAMSys struct {
 	iamCannedPolicyMap map[string]iampolicy.Policy
 }
 
-// Load - load iam.json
+// Load - loads iam subsystem
 func (sys *IAMSys) Load(objAPI ObjectLayer) error {
-	return sys.Init(objAPI)
+	return sys.refresh(objAPI)
 }
 
 // Init - initializes config system from iam.json