add support for global maintenance windows (#3038)

* add support for global maintenance windows
* fix schema validation and trim \ when unmarshalling maintenance window

docs/reference/cluster_manifest.md

@@ -116,9 +116,9 @@ These parameters are grouped directly under  the `spec` key in the manifest.
 
 * **maintenanceWindows**
   a list which defines specific time frames when certain maintenance operations
-  such as automatic major upgrades or master pod migration. Accepted formats
-  are "01:00-06:00" for daily maintenance windows or "Sat:00:00-04:00" for specific
-  days, with all times in UTC.
+  such as automatic major upgrades or master pod migration are allowed to happen.
+  Accepted formats are "01:00-06:00" for daily maintenance windows or
+  "Sat:00:00-04:00" for specific days, with all times in UTC.
 
 * **users**
   a map of usernames to user flags for the users that should be created in the

docs/reference/operator_parameters.md

@@ -173,6 +173,14 @@ Those are top-level keys, containing both leaf keys and groups.
   the thresholds. The value must be `"true"` to be effective. The default is empty
   which means the feature is disabled.
 
+* **maintenance_windows**
+  a list which defines specific time frames when certain maintenance
+  operations such as automatic major upgrades or master pod migration are
+  allowed to happen for all database clusters. Accepted formats are
+  "01:00-06:00" for daily maintenance windows or "Sat:00:00-04:00" for
+  specific days, with all times in UTC. Locally defined maintenance
+  windows take precedence over globally configured ones.
+
 * **resync_period**
   period between consecutive sync requests. The default is `30m`.