Charm should inform about empty file

[Dariusz Smigiel]

When charm reads an empty file, it returns
> WARNING: controller not found

However, there is underlying issue with the file itself, which is empty:

root@compute-server-7:/var/lib/nagios# cat megacli.out
root@compute-server-7:/var/lib/nagios#

Charm should report different information about missing output in a file.

[Drew Freiberger (afreiberger)]

* when the check_$tool script (specifically check_megacli) finds it's input file from cron is blank, it should note that instead of generic warning that the controller is not found.