Want a way to set up mock CNs with a specific platform version

Currently the `mockcloudadm server create` command does not take an argument for the PI version to use to boot up the server. Apparently it is using whatever PI version the head node has (I tried manipulating the version using `sdcadm platform assign` after the fact, or doing `sdcadm platform set-default` before invoking the create command, but neither of them helped).
