Add a QA option to disable reboots during burnin

Since we have seen cases where (repeated) reboots are not supported
(e.g. Xen 3.4+), we need to be able to control this in the QA
configuration.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>