fix secutiryGroup setting, as this is nova-only option

Note that this has been fixed in jclouds 1.8.1 (https://github.com/jclouds/jclouds/commit/e0a6df154422a83cd82dfaf5ece36eebe9784ae2), making Nova use the security groups defined using the portable interface. When upgrading to jclouds 1.8.1, the cast and the check shouldn't be needed.