Switch burnin to cli.JobExecutor

Burnin has a custom job executor, because of its need to retry some job
series.

While we cannot replace all of it, at least the execution we can switch
to cli.JobExecutor, to take advantage of the recently-introduced
out-of-order waiting.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>