Do instance export and import during burnin

Instances get exported to a remote node, then removed and imported back to
their original nodes. This should be an idempotent option from the instance
point of view, and help making sure ImportExport is kept up to date.

It will also help making burnin take a lot longer, which is nice to take a nap.
"...but I'm doing a cluster burnin...". Unfortunately this subfeature is a bit
jeopardized by the fact that the new code can be skipped with the
--no-importexport option, but nobody needs to know that, do they?

Reviewed-by: iustinp