Fix repeated call to Promise::Set() in RunKinit()

Change-Id: I4118d2b6363d0dda29e5ca255f90c38586b1f103
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1527
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: Henry Robinson <henry@cloudera.com>