Move LDAP sync time constant to a method

This moves the `1.hour` constant used as the LDAP sync time to a
separate method. This allows EE to override this method using a module,
instead of directly modifying the source code.

app/models/user.rb

@@ -959,12 +959,17 @@ class User < ActiveRecord::Base
     if !Gitlab.config.ldap.enabled
       false
     elsif ldap_user?
-      !last_credential_check_at || (last_credential_check_at + Gitlab.config.ldap['sync_time']) < Time.now
+      !last_credential_check_at || (last_credential_check_at + ldap_sync_time) < Time.now
     else
       false
     end
   end
 
+  def ldap_sync_time
+    # This number resides in this method so it can be redefined in EE.
+    1.hour
+  end
+
   def try_obtain_ldap_lease
     # After obtaining this lease LDAP checks will be blocked for 600 seconds
     # (10 minutes) for this user.

ee/app/models/ee/user.rb

@@ -156,5 +156,10 @@ module EE
     def has_current_license?
       License.current.present?
     end
+
+    override :ldap_sync_time
+    def ldap_sync_time
+      ::Gitlab.config.ldap['sync_time']
+    end
   end
 end