Test and update arnold backend script for SNMPv3 session compatibilty

[lunkwill42]

Arnold mostly uses the existing PortAdmin management handlers to facilitate port configuration changes, but there is at least one instance of it making its own SNMP session to disable ports:

nav/python/nav/arnold.py

Lines 469 to 473 in e9ae9df

	agent = nav.Snmp.Snmp(
	host=netbox.ip,
	community=profile.snmp_community,
	version=profile.snmp_version,
	)