Reboot command called with full path in Jenkins

OpenSuSE doesn't have /sbin in default user's PATH. Therefore
we can fix a reboot problem with always calling reboot with
its full path.

Change-Id: I276017fc26bf7aca73f601ee0c57941505a2fff0
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Heikki Halmet <heikki.halmet@digia.com>