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

site.yml

@@ -2,23 +2,25 @@
 
 - hosts: k3s_cluster
   gather_facts: yes
-  become: yes
   roles:
     - role: prereq
+      become: true
     - role: download
+      become: true
     - role: raspberrypi
+      become: true
 
 - hosts: master
-  become: yes
   roles:
     - role: k3s/master
+      become: true
 
 - hosts: node
-  become: yes
   roles:
     - role: k3s/node
+      become: true
 
 - hosts: master
-  become: yes
   roles:
     - role: k3s/post
+      become: true