Change default value for SNMP community from 'public' to {$SNMP_COMMUNITY}

When you configuring a new item and choose type SNMPv2, SNMPv1 - a community *public *is suggested as a default. I suggest to change this to {$SNMP_COMMUNITY} - the defacto standard macro used here in all Zabbix SIA and community members SNMP templates to hint new users on what the current best practice is.