allow to configure snmp retries

libnetsnmp by default uses 5 retries if a value request fails. zabbix 2.2 changed default to do no retries.
similar as for icmpping items, it would be nice to configure the amount of snmp retries.

preferably, there would be a global, changeable default, which could be overridden per item (maybe you want retries when connecting to slow, remote switches, but not when monitoring a large, busy local router)

see ZBX-4393 for more discussion