Extend further validation of config values (#8469)

- This PR allows config KVS to be validated properly
  without being affected by ENV overrides, rejects
  invalid values during set operation

- Expands unit tests and refactors the error handling
  for notification targets, returns error instead of
  ignoring targets for invalid KVS

- Does all the prep-work for implementing safe-mode
  style operation for MinIO server, introduces a new
  global variable to toggle safe mode based operations
  NOTE: this PR itself doesn't provide safe mode operations

cmd/admin-handlers_test.go

@@ -90,7 +90,11 @@ func prepareAdminXLTestBed() (*adminXLTestBed, error) {
 	globalPolicySys = NewPolicySys()
 	globalPolicySys.Init(buckets, objLayer)
 
-	globalNotificationSys = NewNotificationSys(globalServerConfig, globalEndpoints)
+	globalNotificationSys, err = NewNotificationSys(globalServerConfig, globalEndpoints)
+	if err != nil {
+		return nil, err
+	}
+
 	globalNotificationSys.Init(buckets, objLayer)
 
 	// Setup admin mgmt REST API handlers.