Support multiple endpoints for snmptrap addr

Hi,

we get more and more requests from customers who ask us to send the SNMP traps to more than a single destination which is currently not supported by Kapacitor.

I see two solutions:
1. addr becomes a list instead of a string and some kind of iteration is used to create the same number of snmp clients via loadNewSNMPClient
2. the snmptrap module can be configured multiple times (like influxdb) and each of them will create a dedicated client

I think the latter solution is the smarter one.

I know this is a real short FR but that's actually it. :)

Start with the snmptrap module and the loadNewSNMPClient entry point, then compare how InfluxDB supports multiple configurations. Decide how multiple SNMP destinations should be configured and verify that each destination receives traps. Done means Kapacitor can send SNMP traps to more than one endpoint.

Written by the indexing model from the issue text.