fix secutiryGroup setting, as this is nova-only option

I'll try to open a PR upgrading to 1.8.1 today/tomorrow.

Also, just FYI, in the next jclouds major release we'll be more flexible with Guava and won't force users to a concrete version. We'll support by default Guava 16.0.1, but we've configured compatibility builds to make sure we also support Guava 17 and 18. Hope this can help removing the shaded dependency!
See: http://markmail.org/message/sqlpgah5vrq5z6op