Update pipeline to use xenial stemcell
Update pipeline to use xenial stemcell

[#161014354]

Signed-off-by: Oleksandr Slynko <4e0b08036a6e8359453f77324c7b401fdae249da@pivotal.io>

cloudfoundry-community/docker-boshrelease,cloudfoundry-community/docker-boshrelease,cf-platform-eng/docker-boshrelease,cf-platform-eng/docker-boshrelease