Create cmd_type packagelist and container_packagelist

[markfeathers]

If a component requires a package, it should be able to define in its manifest.yaml something like:

cmd: packages.sh
cmd_type: packagelist
description: Adding to package

In package.sh, eg, to install an ssh server:

#!/bin/bash
echo "openssh-server"

This script would give an opportunity to echo different package names depending on the distro/release/arch, or any environment variable or config.

The same should happen from a container_packagelist to update a generated dockerfile. This would make it simpler to add container dependencies for packages such as genimage which requires libconfuse from the container.