Local apt defaults

For @howbazaar to correct local environment update/upgrade behavior when bootstrapping.

Running all tests locally now. I won't land unless they pass on my machine, but I wanted to get this up for review.
My preference would be for the .(bool) to go up to the map access:
val, ok := cfgAttrs["enable-os-refresh-update"].(bool)

This will avoid any panics due to cast errors, and sort of fails safe because if there's a problem, we set update to false