seeddb test for snmp version crashes if neither v1 or v2c is supported

[John-Magne Bredal]

Traceback (most recent call last):

  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 109, in get_response
    response = callback(request, *callback_args, **callback_kwargs)

  File "/usr/lib/python2.7/dist-packages/nav/web/seeddb/page/netbox/edit.py", line 86, in get_read_only_variables
    snmp_version = get_snmp_version(ip_address, read_community)

  File "/usr/lib/python2.7/dist-packages/nav/web/seeddb/page/netbox/edit.py", line 150, in get_snmp_version
    snmp = Snmp(ip, community, '1')

  File "/usr/lib/python2.7/dist-packages/nav/Snmp/pynetsnmp.py", line 66, in __init__
    self.handle.open()

  File "/usr/lib/pymodules/python2.7/pynetsnmp/netsnmp.py", line 421, in open
    raise SnmpError('snmp_open')

SnmpError: snmp_open

[John-Magne Bredal (john-m-bredal)]

Fix here: https://nav.uninett.no/hg/stable/rev/7e94cc5a7987