Better validation of all config file fields (#6090)

Add Validate() to serverConfig to call it at server
startup and in Admin SetConfig handler to minimize
errors scenario after server restart.

pkg/event/target/amqp.go

@@ -43,6 +43,17 @@ type AMQPArgs struct {
 	AutoDeleted  bool     `json:"autoDeleted"`
 }
 
+// Validate AMQP arguments
+func (a *AMQPArgs) Validate() error {
+	if !a.Enable {
+		return nil
+	}
+	if _, err := amqp.ParseURI(a.URL.String()); err != nil {
+		return err
+	}
+	return nil
+}
+
 // AMQPTarget - AMQP target
 type AMQPTarget struct {
 	id        event.TargetID