Add PDB configuration toggle (#583)

* Don't create an impossible disruption budget for smaller clusters.
* sync PDB also on update

docs/reference/operator_parameters.md

@@ -161,6 +161,13 @@ configuration they are grouped under the `kubernetes` key.
   replaced by the cluster name. Only the `{cluster}` placeholders is allowed in
   the template.
 
+* **enable_pod_disruption_budget**
+  PDB is enabled by default to protect the cluster from voluntarily disruptions
+  and hence unwanted DB downtime. However, on some cloud providers it could be
+  necessary to temporarily disabled it, e.g. for node updates. See
+  [admin docs](../administrator.md#pod-disruption-budget) for more information.
+  Default is true.
+
 * **secret_name_template**
   a template for the name of the database user secrets generated by the
   operator. `{username}` is replaced with name of the secret, `{cluster}` with