Conform with kubernetes recommended labels

[calind]

https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/. We should set them on cluster subresources and on pods.

• app.kubernetes.io/name -> mysql
• app.kubernetes.io/instance -> MysqlCluster.Labels[app.kubernetes.io/instance] or MysqlCluster.Name
• app.kubernetes.io/version -> image tag
• app.kubernetes.io/component -> MysqlCluster.Labels[app.kubernetes.io/component] or database
• app.kubernetes.io/part-of -> MysqlCluster.Labels[app.kubernetes.io/part-of]
• app.kubernetes.io/managed-by -> mysql-operator.presslabs.org