Support for non-default install options

[mattgeddes]

The installer currently assumes everything based on a detected model config. This falls apart in cases where hardware modifications have been made (we know of some in the field) or for any non-holoport hardware. Also anyone that has non-default preferences. #66 and #65 cover a few specifics, and #112 has some overlap. This issue is to track the remainder. I'll fill in some more detail later, but it's only expected to be a few things to improve usability and supportability.

[mattgeddes]

I'm marking this higher priority -- it's not possible to add wi-fi support or support root password management without having a way to override the installer defaults.