Add a setting for SNMP community lists

[lunkwill42]

This is a feature request from DeIC, based on their experiences with SNMP monitoring using LibreNMS:

When monitoring multiple groups of devices that have different communities, it would nice if the default SNMP community could instead be specified as a list of several community values. For earch router, Zino would try each community in turn, until it finds one that gives a response.

The alternative today is to explicitly configure a community string per device, which gets tedious in this use-case.

[runborg]

also, what about making it possible to add different communities to routers based on eg. subnet, name/"part of name"(regex=) etc?