Bump Ubuntu stemcell back to 456.40
Bump Ubuntu stemcell back to 456.40

[#169430764](https://www.pivotaltracker.com/story/show/169430764)

Co-authored-by: Weyman Fung <4ab5e17be5c5cdce6cf54d6174afda06421ccec1@pivotal.io>

cloudfoundry/cf-deployment,cloudfoundry/cf-deployment