Sync `groups` and `group` to fix validation

Otherwise adding to `groups` will cause validation (in EE)  based on
`group` to fail.

This is consistent with how Clusters::Cluster#first_project. The
alternative is to switch validation to use `groups` as well.

app/models/clusters/cluster.rb

@@ -3,6 +3,7 @@
 module Clusters
   class Cluster < ActiveRecord::Base
     include Presentable
+    include Gitlab::Utils::StrongMemoize
 
     self.table_name = 'clusters'
 
@@ -24,9 +25,6 @@ module Clusters
     has_many :cluster_groups, class_name: 'Clusters::Group'
     has_many :groups, through: :cluster_groups, class_name: '::Group'
 
-    has_one :cluster_group, -> { order(id: :desc) }, class_name: 'Clusters::Group'
-    has_one :group, through: :cluster_group, class_name: '::Group'
-
     # we force autosave to happen when we save `Cluster` model
     has_one :provider_gcp, class_name: 'Clusters::Providers::Gcp', autosave: true
 
@@ -119,12 +117,19 @@ module Clusters
     end
 
     def first_project
-      return @first_project if defined?(@first_project)
-
-      @first_project = projects.first
+      strong_memoize(:first_project) do
+        projects.first
+      end
     end
     alias_method :project, :first_project
 
+    def first_group
+      strong_memoize(:first_group) do
+        groups.first
+      end
+    end
+    alias_method :group, :first_group
+
     def kubeclient
       platform_kubernetes.kubeclient if kubernetes?
     end