The rkt driver currently executes run and asks that the pod UUID is
written to a file that is then polled for changes for up to five
seconds. Many container fetches will take longer than this, so this
method will often not be able to track the pod UUID reliably.
To avoid this problem, rkt allows pods to be first prepared, which will
return their UUID, and then run as a second invocation.
Here we convert the rkt driver's Start method to use this method
instead. This way, the UUID will always be tracked correctly.
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