Add timeout of 6 hours to light stemcell builder job.
Add timeout of 6 hours to light stemcell builder job.

[#109907592](https://www.pivotaltracker.com/story/show/109907592)

cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release