[Linaro-validation] lava-dispatcher bug

Hey Guys,

I found a major bug in our latest lava-dispatcher 0.15 release. This was causing pre-built image testing to not work and cache files in the lab to not get deleted which then led to service outages etc.

Since the issue is huge, the fix is fairly minor, nobody is around, and we are headed into the weekend, I decided just to merge my change without peer review.

Please double check my changes. I've tested them locally and on staging. I'm now deploying to production to hopefully stop the bleeding.

Sorry for breaking all the rules here.

-andy