Feature: Allow users to add custom packages to images built with image-builder

[chrisdoherty4]

As a user, I want to add custom packages to images built with image builder (Ubuntu in particular) during the build process so I can streamline my provisioning process and avoid manually adding packages after the fact.

Packages may require additional configuration steps.

[vignesh-goutham]

The built-in apt ansible package cannot pull deb packages from an http endpoint. On the contrary yum ansilbe package can pull down and install rpms from an http endpoint in addition to pulling them from the repo. Image-builder should patch and maintain a way for users to pull down and install a custom deb package for Ubuntu image builds.

[chrisdoherty4]

Users should be able to execute custom installers.