Allow prompting for (and automatic saving of) configs

This adds a new method, prompt_cfg, which allows you to say "I can't figure out what this value should be, please ask the user".
It then allows the user to specify to save to their user config or not (in which case it will be excluded from ever being saved).
Currently I've implemented it for the initial connection to docker, e.g. if a user doesn't have docker.io, does/doesn't need sudo etc etc.

Additionally, I've cleaned up a few interactive stuff. We now default to interactive level 1, i.e. only pester the user if we can't figure something out (prompt_cfg and pause_points). Pause points now have a 'level' so you don't have to examine cfg['build']['interactive'] directly.