Add downtime constants

db/migrate/20160615142710_add_index_on_requested_at_to_members.rb

 class AddIndexOnRequestedAtToMembers < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
+  DOWNTIME = false
+
   disable_ddl_transaction!
 
   def up

db/migrate/20160620115026_add_index_on_runners_locked.rb

@@ -4,6 +4,8 @@
 class AddIndexOnRunnersLocked < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
+  DOWNTIME = false
+
   disable_ddl_transaction!
 
   def up

db/migrate/20160715134306_add_index_for_pipeline_user_id.rb

 class AddIndexForPipelineUserId < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
+  DOWNTIME = false
+
   disable_ddl_transaction!
 
   def up