roles/nginx: use boolean for use_letsencrypt instead of string "yes"

This is very confusing when you forget about how Ansible/Python is
testing conditionals. Let's use actual booleans so it's more clear.

roles/nginx/templates/https.j2

@@ -31,7 +31,7 @@
     ssl_prefer_server_ciphers on;
 
     {# OSCP stapling only works with real certs #}
-    {% if use_letsencrypt == "yes" or item.tls_certificate_path %}
+    {% if use_letsencrypt == True or item.tls_certificate_path %}
     # OCSP stapling...
     ssl_stapling on;
     ssl_stapling_verify on;