Merge pull request #16731 from carbonin/validate_zone_name_within_region

Ensure that the zone name is unique only within the current region
(cherry picked from commit 1bf5290)

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1530820

app/models/zone.rb

@@ -1,6 +1,6 @@
 class Zone < ApplicationRecord
   validates_presence_of   :name, :description
-  validates_uniqueness_of :name
+  validates :name, :unique_within_region => true
 
   serialize :settings, Hash