Check K3s installed version before download tasks (#297)

- [Agent : Download artefact only if needed](roles/k3s_agent/tasks/main.yml#L13)
- [Server : Download artefact only if needed](roles/k3s_server/tasks/main.yml#L13)
- [Upgrade : Upgrade node only if needed](roles/k3s_upgrade/tasks/main.yml#L14)

Linked issue #264 k3s_server and k3s_agent tasks are not idempotent

Signed-off-by: Loïc Dubard <loic97429@gmail.com>