Add checkout flow scenarios

https://github.com/balanced/balanced-api/blob/revision1/scenarios/credits/credit_card_fail.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/credits/credit_customer.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/credits/credit_customer_fail.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/checkout_flow/cancel_an_order.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/checkout_flow/existing_buyer_purchases_with_a_new_card.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/checkout_flow/existing_buyer_purchases_with_an_existing_card.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/checkout_flow/new_buyer_purchases_an_item.yml