GUI-1460 Eucalyptus VPC adjustments

https://eucalyptus.atlassian.net/browse/GUI-1460
@kyolee310: Default arg values should never be mutable.  See http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments

To fix, change the arg to `attribute_names=None` and add the following line below line 281…
`attribute_names = attribute_names or ['supported_platforms']`