Security exposure related to the token (#356)

* Security exposure related to the token

The installation playbook saves the token into the systemd unit
configuration file /etc/systemd/system/k3s.service. The problem is that
according to K3s' documentation "the server token should be guarded
carefully" (https://docs.k3s.io/cli/token), yet the configuration file
is readable by anybody. A better solution is to save the token into its
corresponding environment file /etc/systemd/system/k3s.service.env which
is readable by the super user only. This is what the standard K3s'
installation script (https://get.k3s.io) does.

Signed-off-by: Marko Vukovic <8951449+anon-software@users.noreply.github.com>

* Restore the server URL into systemd configuration file

There aren't any security implications in keeping it there.

Signed-off-by: Marko Vukovic <8951449+anon-software@users.noreply.github.com>

---------

Signed-off-by: Marko Vukovic <8951449+anon-software@users.noreply.github.com>

roles/k3s_agent/tasks/main.yml

@@ -35,6 +35,14 @@
         INSTALL_K3S_EXEC: "agent"
       changed_when: true
 
+    - name: Add the token for joining the cluster to the environment
+      no_log: true # avoid logging the server token
+      ansible.builtin.lineinfile:
+        path: "{{ systemd_dir }}/k3s-agent.service.env"
+        line: "{{ item }}"
+      with_items:
+        - "K3S_TOKEN={{ token }}"
+
 - name: Copy K3s service file
   register: k3s_agent_service
   ansible.builtin.template: