LTM to use/handle preemptible instances

[tytso]

As the LTM is monitoring test VMs, it is possible that the child VMs are run in preemptible instances.

This will require the LTM to be able to detect the difference between preemption and successful test completion, and re-launch the child VM with all unfinished tests, starting from the last known test completion. This will also need an improvement to the results aggregation, as the results from the preempted child test appliance will need to be kept, and combined with the results of the resumed test appliance.

This may require a change to xfstests (an upstream package) to include the ability to "resume" an FSTESTSET from a given point, or to implement this "resume tests" behavior separately using custom FSTESTSETs. See: #9