graph: Allow provisioning users with legacy names (#5255)

Via configuration you can now configure to skip the validation of username and
instead decide to trust the upstream system that is adding users.

services/graph/pkg/config/parser/parse.go

@@ -52,5 +52,15 @@ func Validate(cfg *config.Config) error {
 			"graph", defaults2.BaseConfigPath())
 	}
 
+	switch cfg.API.UsernameMatch {
+	case "default", "none":
+	default:
+		return fmt.Errorf("The username match validator is invalid for %s. "+
+			"Make sure your %s config contains the proper values "+
+			"(e.g. by running ocis init or setting it manually in "+
+			"the config/corresponding environment variable).",
+			"graph", defaults2.BaseConfigPath())
+	}
+
 	return nil
 }