At present, the OpenStack cloud plugin allows Jenkins to automatically create new slave executors by provisioning new OpenStack instances (virtual machines) from an OpenStack image.
OpenStack also supports the creation of instances from an OpenStack "volume snapshot" (see http://www.openstack4j.com/learn/compute/servers/), but the plugin currently has no support for this (it only supports images).

I want the option of creating new OpenStack instances from an OpenStack volume snapshot.
i.e. allow users to EITHER specify an image OR specify a volume snapshot.