Make setting enable_proxy_headers_parsing optional

This option specifies whether service is running under proxy

Change-Id: I92963d153720672e90db6e0444cc4784aabb9e69
diff --git a/gnocchi/files/4.0/gnocchi.conf b/gnocchi/files/4.0/gnocchi.conf
index 3346841..1658172 100644
--- a/gnocchi/files/4.0/gnocchi.conf
+++ b/gnocchi/files/4.0/gnocchi.conf
@@ -122,7 +122,9 @@
 
 [oslo_middleware]
 
+{%- if server.enable_proxy_headers_parsing is defined %}
 enable_proxy_headers_parsing = {{ server.enable_proxy_headers_parsing }}
+{%- endif %}
 
 [statsd]