refactor(build_library): Add a function for enabling systemd units.
feat(build_image): Add support for injecting a cloud config

The new --developer_data option can be used to specify a path to a cloud
config to bundle into the image. If none is provided but a shared user
password (for core) is set then generate a config to set that password.
This lets us use the same mechanism for setting the default password for
both disk and PXE images.