Empties bucket before attempting to delete stack
Empties bucket before attempting to delete stack

[#115708761](https://www.pivotaltracker.com/story/show/115708761)

Signed-off-by: Edwin Xie <6bc24e9c160281972837d9e13287696d26612564@pivotal.io>

pivotal-golang/s3cli,pivotal-golang/s3cli,pivotal-golang/s3cli,pivotal-golang/s3cli