Let the soft reboot really happen in the minimum scenario

The cirros image can support the soft reboot, but we reboot the machine
before the acpid started, it leads to additional 120 second wait before
nova falls back to the hard reboot.
As a solution we will ssh the machine before reboot, because the ssh
server starts after the acpid, so we will send the ACPI event, only
when the acpid is really running.

Change-Id: I3700a374dc937086518c5c843a288e935ab458bf
Merge "Let the soft reboot really happen in the minimum scenario"