Introduce new 'downloading' State for instances

[markdryan]

If you start a docker container on a compute node which uses a base image that has not previously been used by any container on that compute node, ciao-launcher needs to download the image. This can greatly increase the start time of instance. We should therefore add a new state to indicate to the users why the image start is taking so long. 'downloading' should do the trick.

[rbradford]

This is no longer required as the images are stored in ceph and so there is no need download the image and so no need for this new state.

[markdryan]

The docker images aren't stored in ceph though. These are still downloaded to the local compute node before a container is launched.