apache: on service refresh, do a graceful reload instead of start/stop

With this change, when Puppet refreshes Service['apache2'], it'll initiate a
graceful reload rather than a full service stop / service start. A graceful
reload allows open requests to terminate gracefully, and it performs a config
check before shutting down, so it comes with the added benefit of not leaving
the service dead in the water if you happen to deploy a config change with a
syntax error.

Change-Id: Id030fef0016fdc07076bb1d375e7b9b2c9c5229d