Call scap-rebuild-cdbs after sync-common

The existing eventual consistency system for ensuring that the MediaWiki
deployment on a host is up to date doesn't ensure that the localization
cache is updated. This divergence was introduced in I0b18af5 when the
l10n cache update process was decoupled from sync-common.

For those trying to follow along at home, Exec['mw-sync'] is notified
from the applicationserver::service class if apache2 is not running at
the time that class is applied to the host. This would typically happen
on a cold boot of the host. When the Exec['mw-sync'] is finished it will
notify Service['apache'] to start the apache2 process.

This patch introduces a new Exec['mw-sync-rebuild-cdbs'] which also
subscribes to Exec['mw-sync']. This is not perfect as the ordering of
Service['apache'] and Exec['mw-sync-rebuild-cdbs'] being applied is not
guaranteed, but it would ensure that the l10n cache is updated soon
after apache2 starts if not before.

Change-Id: I6c433373d24912a1e752b7e4a08eb01e0332de72