Refactor config to match ACS guidelines

We're using top-level keys for k3s like k3s_version. The new preferred layout is:

k3s:
  version: "v1.21.1+k3s1"
  token: "mysupersecretoken"
  agent:
    extra_args: ""
  server:
    extra_args: "--disable traefik,local-storage --disable-cloud-controller --node-taint CriticalAddonsOnly=true:NoExecute"
  apiserver:
    endpoint: "{{ hostvars[groups['master'][0]]['ansible_host'] | default(groups['master'][0]) }}"

• refactor defaults.yml
• provide backwards compatibility (by using the old keys as default sources; this helps supporting older configs)
• refactor roles to use rc.k3s as base
• refactor roles so only one role named k3s will be executed, matching app behavior