server: add configurable cache size, with correct default

zabbix-server/defaults/main.yml

+---
+
+zabbix_cache_size: 8M

zabbix-server/templates/zabbix_server.conf.j2

@@ -69,7 +69,7 @@ LogFileSize=0
 # Default:
 # PidFile=/tmp/zabbix_server.pid
 
-PidFile=/var/run/zabbix/zabbix_server.pid
+PidFile=/run/zabbix/zabbix_server.pid
 
 ### Option: DBHost
 #	Database host name.
@@ -351,7 +351,7 @@ StartDiscoverers= {{ startdiscovers }}
 # Mandatory: no
 # Range: 128K-8G
 # Default:
-# CacheSize=8M
+CacheSize={{ zabbix_cache_size }}
 
 ### Option: CacheUpdateFrequency
 #	How often Zabbix will perform update of configuration cache, in seconds.