Replace non working option by a jinja2 snippet

Asking to gluster devs, they were surprised this would be
working in the first place.

roles/gluster/client/templates/client.config

@@ -38,7 +38,7 @@ end-volume
 
 volume iocache
 	type performance/io-cache
-	option cache-size `grep 'MemTotal' /proc/meminfo  | awk '{print $2 * 0.2 / 1024}' | cut -f1 -d.`MB
+	option cache-size {{ (ansible_memtotal_mb / 5) |round |int }}MB
 	option cache-timeout 30
 	subvolumes iothreads
 end-volume