Verify node images before cluster deployment

[jefflill]

I just ran into a situation where I was installing a XenServer cluster and the node image installation step failed due to reaching the EOF expectantly. I believe this was due to canceling a cluster prepare earlier while the node image was still downloading. Deleting the local cached node images fixed this.

We should probably do a sanity check in cluster prepare to verify that the node image is OK. Just verifying the size would probably be good enough to avoid the time it would take to compare MD5 hashes.

We should remove the local node image and re-download it when it looks bad.