No longer using stemcell-based warden root FS; warden must bring its own

/cc @mkocher

It looks like the root FS comes from other locates now:

* https://github.com/cloudfoundry/warden-test-infrastructure/blob/master/ci-cookbooks/warden/recipes/default.rb#L3

*BUT* the dea_next job still looks like it expects `/var/vcap/stemcell_base.tar.gz` to exist :(

* https://github.com/cloudfoundry/cf-release/blob/master/jobs/dea_next/templates/warden_ctl#L28

This needs to be fixed before this PR is merged I guess.