Add `miren runner remove` command to deregister stale nodes

From MIR-903 #17.

miren runner revoke is for invite codes, not registered nodes. There's no way to clean up stale node entries from etcd. After repeated join attempts, the runner list accumulates stale entries that will never come back.

Need miren runner remove <node-name> that deletes the node entity from etcd. Should also clean up any associated resources (flannel leases, etc.).