Some fixes

[galindro]

• FIX #1: Removed opts.host variable to fix the issue pointed by http://www.zabbix.org/wiki/Docs/howto/apache_monitoring_script#Method_1
• FIX #2: Check if zabbix_sender binary exists before try to use it. If not exists or if it isn't executable, the script will log the issue in syslog and raise an exception

[gpmidi]

Thanks Galindro