Add boolean auth parameter `allow_read`

Boolean auth parameter `allow_read` can be used to permit read access for
not authorized users.

  site:
    restricted_site:
      ...
      auth:
        allow_read: true
        engine: basic

Change-Id: I23fd2f74c5b1eb29b7a96b3d1432768326bdc0de
diff --git a/nginx/files/_auth.conf b/nginx/files/_auth.conf
index 2cc3653..0db8d42 100644
--- a/nginx/files/_auth.conf
+++ b/nginx/files/_auth.conf
@@ -1,5 +1,9 @@
 
   {%- if site.auth is defined %}
+  {%- set allow_read = site.auth.get('allow_read', False) %}
+  {%- if allow_read  %}
+  limit_except GET {
+  {%- endif %}
   {%- if site.auth.engine == 'basic' %}
   auth_basic "Restricted";
   {%- if site.auth.htpasswd is defined %}
@@ -8,4 +12,7 @@
   auth_basic_user_file /etc/nginx/htpasswd;
   {%- endif %}
   {%- endif %}
+  {%- if allow_read %}
+  }
+  {%- endif %}
   {%- endif %}