Add repack-google-stemcell script
Add repack-google-stemcell script

- Lets you repack a stemcell with a customized bosh-agent

Co-authored-by: Aakash Shah <88d21716b3df1e9d291d91c646300964a4b6d807@pivotal.io>
Co-authored-by: Merric de Launey <79964dcbacbf8006d1234555614dad3bcf49e6d6@pivotal.io>

gu-bin/bosh-agent,mattcui/bosh-agent,mattcui/bosh-agent,cloudfoundry/bosh-agent,gu-bin/bosh-agent,gu-bin/bosh-agent,cloudfoundry/bosh-agent,mattcui/bosh-agent