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/config/identity/openid/validators_test.go

@@ -87,7 +87,7 @@ func TestValidators(t *testing.T) {
 		t.Fatalf("Unexpected number of vids %v", vids)
 	}
 
-	u, err := xnet.ParseURL(ts.URL)
+	u, err := xnet.ParseHTTPURL(ts.URL)
 	if err != nil {
 		t.Fatal(err)
 	}