Check fullchain.pem, not cert.pem

While using certbot means we'll have both files retrieved,
it's actually the fullchain.pem file that we use in nginx configuration.

Using that one for the check makes more sense.

roles/matrix-nginx-proxy/tasks/ssl/setup_ssl_lets_encrypt_obtain_for_domain.yml

@@ -2,7 +2,7 @@
     msg: "Dealing with SSL certificate retrieval for domain: {{ domain_name }}"
 
 - set_fact:
-    domain_name_certificate_path: "{{ matrix_ssl_config_dir_path }}/live/{{ domain_name }}/cert.pem"
+    domain_name_certificate_path: "{{ matrix_ssl_config_dir_path }}/live/{{ domain_name }}/fullchain.pem"
 
 - name: Check if a certificate for the domain already exists
   stat: