Container type error

Fix an error when trying to create a container and the type has not been selected.

https://bugs.launchpad.net/juju-gui/+bug/1350697
One thing to note is that usually there's only one valid container type. In AMZ it's only LXC, from an LXC it's only KVM. We need to get those rules down and I think the UX problem will fall into an easier place.