Add default REBOOT_TIMEOUT to system tests

Change-Id: I4db420c8e85955dac71dbf5216412ad24b96f405
Related-bug: #1365345
(cherry picked from commit b344bb686f77b7eebe6adba9aa0e15e0e4405df5)
Merge "Add default REBOOT_TIMEOUT to system tests" into stable/5.1