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/storage-rest_test.go

@@ -498,7 +498,7 @@ func newStorageRESTHTTPServerClient(t *testing.T) (*httptest.Server, *storageRES
 	router := mux.NewRouter()
 	httpServer := httptest.NewServer(router)
 
-	url, err := xnet.ParseURL(httpServer.URL)
+	url, err := xnet.ParseHTTPURL(httpServer.URL)
 	if err != nil {
 		t.Fatalf("unexpected error %v", err)
 	}