Decrease reviewday update frequency.

* modules/reviewday/manifests/site.pp: The frequency of the update
reviewday cron job is now every half hour instead of every quarter
hour. Additionally, a lock is added to prevent overlapping runs in
case the script begins to take longer. This is needed since recent
Launchpad API changes have caused it to perform considerably slower
than before.

Change-Id: I9fcdbb350a61ab29200068406ccb8b8eaa5e4244
Reviewed-on: https://review.openstack.org/28331
Reviewed-by: Dan Prince <dprince@redhat.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins