Update SeedDB writeable profile connectivity verification to support SNMPv3

[lunkwill42]

The SeedDB connectivity tests have their own mode for testing that an SNMP profile marked as writeable will actually allow a write command. This was not updated in #2704 as we did not yet have an SNMP entity that was configured with a write-enabled user.

This is mostly about updating this function:

nav/python/nav/web/seeddb/page/netbox/edit.py

Lines 188 to 219 in e9ae9df

	def snmp_write_test(ip, profile):
	"""Test that snmp write works"""
	testresult = {
	'error_message': '',
	'custom_error': '',
	'status': False,
	'syslocation': '',
	}
	syslocation = '1.3.6.1.2.1.1.6.0'
	value = ''
	try:
	snmp = Snmp(
	ip,
	profile.configuration.get("community"),
	profile.configuration.get("version"),
	)
	value = safestring(snmp.get(syslocation))
	snmp.set(syslocation, 's', value.encode('utf-8'))
	except SnmpError as error:
	testresult['error_message'] = error.args
	testresult['status'] = False
	except UnicodeDecodeError as error:
	testresult['custom_error'] = 'UnicodeDecodeError'
	testresult['error_message'] = error.args
	testresult['status'] = False
	else:
	testresult['status'] = True
	testresult['syslocation'] = value
	return testresult