Set /.well-known/acme-challenge nginx location path as prefix string (#192)

In nginx.tmpl, vhosts.d are included before the 'location /' target, and last basic location wins.
If our /.well-known location is defined as a prefix string (or as a regex), it takes priority over basic locations
Details on http://nginx.org/en/docs/http/ngx_http_core_module.html#location

Author: lounagen

app/nginx_location.conf

@@ -1,4 +1,4 @@
-location /.well-known/acme-challenge/ {
+location ^~ /.well-known/acme-challenge/ {
     allow all;
     root /usr/share/nginx/html;
     try_files $uri =404;