Add SNMP community and port variables in SNMP test item window

When testing SNMP items, a lot of problems are associated with wrong SNMPv2 community or SNMPv3 security credentials

I suggest adding additional fields on item test form:

• community name and port for SNMP v1, SNMP v2c
• context name, security name and port for SNMP v3

Also consider, that this is sensitive information so the user macros containing this information may be protected.