GUI-1460 Eucalyptus VPC adjustments

https://eucalyptus.atlassian.net/browse/GUI-1460
@kyolee310  The in statement check assumes a list, but if `request.session.get('supported_platforms')` doesn't fetch anything (e.g. in unit tests), it returns None here.  For a safer lookup, we'll need to change this line to the following…
```
return 'VPC' in request.session.get('supported_platforms', [])
```

Fixing this here should allow our Travis CI tests to pass AFAICT