Make cluster restart safe

implemented database cluster restart with kubectl rollout restart. It is good enough for alpha, but not good enough for beta: we can lose data this way, and operators' team confirmed it is not safe.

For beta1, we should implement restart via full cluster pause/resume: https://www.percona.com/doc/kubernetes-operator-for-pxc/pause.html

The same functionality will be available for PSMDB.