Install harbor metal key on experiment demat

When an experiment is dematerialized, the /metal/<node> etcd key is deleted. This means services such as sledapi that track metal keys do not know what to do with a node after it is dematerialized. This change updates the apiserver to reinstall the harbor mtz data in the /metal/<node> key any time an experiment using is dematted.

The implementation uses a separate etcd key /harbor.metal when the harbor is initialized. On demat, the contents are copied from /harbor.metal/<node> to /metal/<node>. We should be able to use key revisions to do this and not use a new key, but Mars storage package currently lacks the necessary scaffolding around etcd usage/transactions needed to track revisions