Protect against faulty SNMP profiles

ManagementProfile.snmp_version now avoids crashing in the case of
faulty SNMP profiles.  Since it was used to sort multiple profiles
in order of SNMP version while fetching preferred profiles, the
presence of any faulty profile on a Netbox would make
`get_preferred_snmp_profile()` unable to fetch any valid profile without
crashing.

This was discovered by our test suite.

python/nav/models/manage.py

@@ -160,10 +160,17 @@ def is_snmp(self):
     def snmp_version(self):
         """Returns the configured SNMP version as an integer"""
         if self.protocol == self.PROTOCOL_SNMP:
-            value = self.configuration['version']
+            value = self.configuration.get("version")
             if value == "2c":
                 return 2
-            return int(value)
+            if value:
+                return int(value)
+            else:
+                _logger.error(
+                    "Broken management profile %s has no SNMP version", self.name
+                )
+                return None
+
         elif self.protocol == self.PROTOCOL_SNMPV3:
             return 3