Add an internal test method for rebooting a server

test_reboot_server_hard and test_reboot_server_soft contain almost
the same code and they are redundant.
This patch adds an internal test method and makes them use the method
for cleanup.

Change-Id: I98fc519b5bb2de8cc47007ba9138247f4c0f3369
Merge "Add an internal test method for rebooting a server"