root_fs now comes from rootfs_lucid64 package

/var/vcap/stemcell_base.tar.gz is being removed from the base stemcell

This PR goes together with https://github.com/cloudfoundry/bosh/pull/152
but can be merged independently. The /var/vcap/stemcell_base.tar.gz is
not being used by dea_next/warden anymore.