Add helper for finding unclaimed drives

This is a prereq for cinder to check if any disk-backed volume can be
created.
Handle removed node referenced by another proposal

When another proposal removes a node which the current proposal
references, the corresponding DOM element is no longer available in the
deployment list. However, info from this element is needed by the
nodeList, when it triggers an event that a node has been removed.

Make the nodeList look up the info in some other way.
Merge pull request #1001 from bkutil/unclaimed_drives_helper

Add helper for finding unclaimed drives
Merge pull request #1003 from bkutil/handle_nodes_removed_from_role

Handle removed node referenced by another proposal