Support upstart-based images and make "apt-get update" optional

These changes are motivated by the "machine-like" image use case supporting Upstart under Ubuntu (e.g. `ubuntu-upstart`), where diverting `/sbin/initctl` is undesirable. 

Also, `apt-get update` takes a bit of time, and makes it possible to disable it (e.g. if it is already invoked from Chef).