Do not rely on ansible_user (#214)

* Apply "become" on roles instead of plays

This leads to facts being gathered for the "regular" login user, instead
of root.

* Do not rely on ansible_user

Instead of reading ansible_user (which may or may not be defined), this
patch lets the roles rely on Ansible facts [1].

[1]: https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_vars_facts.html

roles/k3s/master/defaults/main.yml

@@ -1,5 +1,4 @@
 ---
-ansible_user: root
 server_init_args: >-
   {% if groups['master'] | length > 1 %}
     {% if ansible_hostname == hostvars[groups['master'][0]]['ansible_hostname'] %}