Move Image Generation Scripts into Plugins

As Sahara matures, it will be necessary to move framework-specific plugins
into their own repositories, maintaining a backward-compatible Sahara core
separately. While there are other steps which will make this more feasible
(including modularizing data sources and engines, such that different
categories of plugins can depend on one another without friction.) However,
perhaps the most critical change needed to permit splitting plugins from
the Sahara core repository is to move image generation architecture into
the plugins themselves. As plugins (hopefully!) proliferate, using a
variety of configuration and image generation strategies, we will no longer
be able to maintain all image generation processes in a single, monolithic
repository which demands a single image generation strategy
(disk-image-builder.) As such, moving image generation logic into plugins
is a critical piece of and precondition for this abstraction effort.