ci: bump stemcell to xenial
ci: bump stemcell to xenial

cloudfoundry-incubator/bosh-google-cpi-release,cloudfoundry-incubator/bosh-google-cpi-release,evandbrown/bosh-google-cpi-release,evandbrown/bosh-google-cpi-release,evandbrown/bosh-google-cpi-release,cloudfoundry-incubator/bosh-google-cpi-release,evandbrown/bosh-google-cpi-release,cloudfoundry-incubator/bosh-google-cpi-release