WIP: permit missing/empty vars.

This allows for scripts to be passed through directly to the post-processor, without templating.
vm_config[:bootstrap], not vm_config[:bootstrap_config]
PR feedback: No bootstrap config means no preamble processing.

Just pass an empty string to Fog for the host customization.
Minor typo fixes.
No need to rescue preamble exceptions now.

We want these to bubble up if we hit them.

Following PR review with @mattbostock, preamble processing is no longer triggered if bootstrap configuration is absent (thanks to @rjw1). Other issues should be raised to the caller.