* Decouple role kubespray-defaults from download
Avoids doing re-importing the download role on every invocation of
kubespray-defaults (and skipping everything).
This has a measurable effect on playbook performance.
* Update docs refering to moved download defaults
All container image versions were defined in download/defaults/main.yml
except containerd.
The inconsistency caused the offline script(generate_list.sh) could not
output the URL of containerd image.
This moves the definition into a valid file.
In addition, this adds host_os to generate_list.sh for downloading
krew from a valid URL.
Louis Tu
Max Gautier
Victor Morales
Kenichi Omichi
Takuya Murakami
muzi502