Add config file

Support non-interactive running of the installer by providing a config file that just defines the shell variables that the interactive steps fill out.

This doesn't avoid showing the UI - e.g. progress bars will still be shown.
It just makes it noninteractive. 

It is also possible to only pre-set a few variables. The installer will prompt for any unspecified ones, as before.

Also includes a sample config script with comments, available to the user by calling 'cloud-install -e' (e for example).