roles/nginx: Parameterize HSTS header

This parameterizes the HTTP Strict Transport Security header so we
can use it consistently across all templates. Also, it updates the
max-age to be ~1 year in seconds, which is recommended by Google.

See: https://hstspreload.org/

roles/nginx/templates/https.j2

@@ -51,5 +51,5 @@
     # Enable this if you want HSTS (recommended, but be careful)
     # Include all subdomains and indicate to Google that we want this pre-loaded in Chrome's HSTS store
     # See: https://hstspreload.appspot.com/
-    add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload" always;
+    add_header Strict-Transport-Security "max-age={{ nginx_hsts_max_age }}; includeSubDomains; preload" always;
     {% endif %}