Fix gen-stemcell to support new stemcell-builder

Change gen-stemcell to use StageRunnder because build_from_spec.sh was removed from
bosh.
https://github.com/cloudfoundry/bosh/commit/34b4ca5d5350f461a0e413c9416aa3b7135a8542
Merge pull request #19 from jacopen/master

Fix gen-stemcell to support new stemcell-builder