refactor(k3s): centralize k3s primary server IP and integrate Netcup DNS

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

roles/k3s_loadbalancer/vars/main.yml

@@ -1 +1,7 @@
 k3s_loadbalancer_nginx_config_path: "/etc/nginx/nginx.conf"
+
+k3s_loadbalancer_netcup_api_key: "{{ netcup_api_key }}"
+k3s_loadbalancer_netcup_api_password: "{{ netcup_api_password }}"
+k3s_loadbalancer_netcup_customer_id: "{{ netcup_customer_id }}"
+
+domain: "{{ internal_domain }}"