chore(app-emulation/docker): remove symlink-usr case

We are never shipping another -generic image so just make symlink-usr
the only .service file.
fix(app-emulation/docker): add TMPDIR=/var/tmp

This fixes the issue reported here where /tmp runs out os space during a
docker save

https://github.com/coreos/coreos-vagrant/issues/49#issuecomment-43287780