Fix for Fotigate devices using VDOMs

I had problems snapshotting Fortigate devices that use VDOMs. I found an [old thread](http://www.shrubbery.net/pipermail/rancid-discuss/2010-January/004565.html) where someone had the same issue but never found a solution.

To get it working properly, rancid first needs to run `config global` when multiple VDOMs exist on the device. I added a check for  the string `show` and, if it is not present, run `config global`. After that, everything works the same as a Fortigate device without VDOMs.