keyholder: fix `keyholder` script

* Make the mwdeploy key group-readable and chgrp'd to keyholder, so that we can
  read it when we sudo to keyholder to add it to the agent.
* Consolidate list, list-proxy and status subcommands into status.

Change-Id: I0e71c1f79c6a7313ecd6085165d5eba06524bed1
Merge "keyholder: fix `keyholder` script" into production