packer: fix ansible_os_family and iso_checksum

[dongsupark]

It has not been possible to create images for other distros like Ubuntu 1804, because our Flatcar PR started to specify ansible_os_family in packer/config/ansible-args.json.
It means, it is not able to get the OS family recognized by Ansible.
We need to manually specify ansible_os_family in each distro-specific config.

Now that iso_checksum is composed of iso_checksum_type:iso_checksum_value, we need to change the format of iso_checksum in Flatcar.
Since the value relies on the format of file:https://..., we need to move the type part file into a new field iso_checksum_type, which is now required by packer/qemu/packer.json. Then we do not need to keep the prefix file: in iso_checksum.

[dongsupark]

Now the upstream image-builder has changed code around goss.
Rebasing the whole tree on top of master is not trivial. Fun.
Will soon the rebase and tests together with this PR.

[dongsupark]

Ready for review.

Pushed also fixes for the iso_checksum issue.

Note, the commit capi: remove iso_checksum_type in qemu for packer 1.6 is already created as a separate PR to upstream, kubernetes-sigs#289. So after the PR got merged by upstream, we will delete the commit.

[dongsupark]

Note, the commit capi: remove iso_checksum_type in qemu for packer 1.6 is already created as a separate PR to upstream, kubernetes-sigs#289. So after the PR got merged by upstream, we will delete the commit.

Now that the PR kubernetes-sigs#289 was merged, I dropped the commit capi: remove iso_checksum_type in qemu for packer 1.6.
Tested again. It works.

[dongsupark]

Merged to kubernetes-sigs#248.