Add alien-image-list command

To list images about which nodepool has no knowledge.

Change-Id: I6c861abaf6b53a8e50d730590eaa3509ca864f61
Add a hold command

To put a node into the HOLD state.

Change-Id: I2f173ce928f4ff8399c6fdc00d03a09fa3b20136
Add a delete command

To delete a node.

Change-Id: Ifd2639b1bfd069807c64d95717822f469a6601fb
Add image-delete command

To delete an image.

Change-Id: Ied9ac09073780c63201aaf2fcd6fa1e995c1c6ab