Add a util function for SNMP
Add a util function for SNMP

On the path to instrumenting network switches, first
we'll add some framework for SNMP.  Given that we are
using eventlet and thus we need a patchable SNMP,
we employ PySNMP, despite it being a bit peculiar.
This commit tucks away the oddness and makes it
pretty easy to use for our purposes.

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