refactor(k3s): streamline inventory and primary server IP handling

Signed-off-by: Tuan-Dat Tran <tuan-dat.tran@tudattr.dev>

roles/k3s_agent/tasks/installation.yml

@@ -16,6 +16,6 @@
   ansible.builtin.command: |
     /tmp/k3s_install.sh
   environment:
-    K3S_URL: "https://{{ k3s.loadbalancer.ip }}:{{ k3s.loadbalancer.default_port }}"
+    K3S_URL: "https://{{ hostvars['k3s-loadbalancer'].ansible_default_ipv4.address }}:{{ k3s.loadbalancer.default_port }}"
     K3S_TOKEN: "{{ k3s_token }}"
   become: true