Add a default serial on handler restart

defaults/main.yml

@@ -8,6 +8,7 @@ mysql_vendor: 'mariadb'
 mysql_root_password: 'change_me_NOW'
 mysql_notify_restart: true
 mysql_upstream_apt_src: false
+mysql_serial_restart: 1
 
 # -------------------------------------
 # Configuration

handlers/main.yml

@@ -3,3 +3,4 @@
 - name: restart mysql
   action: service name=mysql state=restarted enabled=yes
   when: mysql_notify_restart
+  serial: {{ mysql_serial_restart }}