[Linaro-validation] Staging health checks

Hi all,

I've just checked in and deployed a more general fix for the network failures on staging, and it's running through its paces now. Essentially, the change now tries to reboot three times if *anything* goes wrong, not just the network coming up. Looking back at the failures of the last 24 hours (2 out of 158) both of those would have been fixed by my change.

We'll know how well we've done in 24 hours time.

Thanks

Dave