Ssh fingerprint fix

Created by: raphendyr

We should check exit status instead of look sting n output.

Someone need to validate the tests as I wrote them without testing...

I tested manually the code, so it works.

This should be merged to 4-1-stable and 4-2-stable too, as I think this is security update.

If this patch is not used, users might get gitolite (or any other) system in blocked state.