Add a new contrib script, wallet-rekey-periodic, which is used at
Stanford to periodically rekey hosts from cron.

Change-Id: Ic1f515da44e55623f7d6864f9a3cebf24c08e13b
Reviewed-on: https://gerrit.stanford.edu/1547


Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>