nginx/server: Use SNI when connecting to backend servers

roles/nginx/server/templates/nginx.conf.j2

@@ -16,6 +16,7 @@ http {
     proxy_ssl_certificate {{ tls_certs }}/{{ inventory_hostname }}.crt;
     proxy_ssl_certificate_key {{ tls_private }}/{{ inventory_hostname }}.key;
     proxy_ssl_trusted_certificate {{ tls_certs }}/ca.crt;
+    proxy_ssl_server_name on;
     proxy_ssl_verify on;
 
 {% if plaintext is defined %}