poweroff after installation

In order to copy-in the SSH key, we need to be off.

preseed.cfg

@@ -390,8 +390,10 @@ d-i finish-install/reboot_in_progress note
 # This is how to make the installer shutdown when finished, but not
 # reboot into the installed system.
 #d-i debian-installer/exit/halt boolean true
+
 # This will power off the machine instead of just halting it.
-#d-i debian-installer/exit/poweroff boolean true
+# In order to copy-in the SSH key, we need to be off.
+d-i debian-installer/exit/poweroff boolean true
 
 ### Preseeding other packages
 # Depending on what software you choose to install, or if things go wrong