[drainless] delay reboot process for 5s to ensure connection termination

Occastionally reboot operation can fail due to connection is still not
finished before reboot happens. This make sure that the client receives
the response of the reboot.

Tested: reboot is delayed.

Google-Bug-Id: 518836174
Change-Id: I42d44bc0d022091a47479309a9592c02a6fbd549
Signed-off-by: Yuxiao Zhang <yuxiaozhang@google.com>