Add a workaround for the shutdown race condition

http://comments.gmane.org/gmane.linux.arch.general/48739

Basically systemd kills the networking before it kills ssh sessions, and
Packer will try and wait for a 0 exit code after the shutdown command to
confirm that it worked. Things happen too quickly with systemd, and so
Packer complains when gracefully halting the machine that:

    "Build 'vmware' errored: Shutdown command has non-zero exit status."

This workaround just adds a standard systemd timer to the poweroff
command so it has a one second delay, which is long enough for the
0 exit code to return and signal a successful graceful halt.

arch-template.json

@@ -14,7 +14,7 @@
             "disk_size": 20480,
             "ssh_username": "root",
             "ssh_password": "vagrant",
-            "shutdown_command": "systemctl poweroff"
+            "shutdown_command": "systemctl start poweroff.timer"
         }
     ],
     "post-processors": [

install.sh

@@ -66,6 +66,8 @@ cat <<-EOF > "${TARGET_DIR}${CONFIG_SCRIPT}"
 	/usr/bin/curl --output /home/vagrant/.ssh/authorized_keys https://raw.github.com/mitchellh/vagrant/master/keys/vagrant.pub
 	/usr/bin/chown vagrant:users /home/vagrant/.ssh/authorized_keys
 	/usr/bin/chmod 0600 /home/vagrant/.ssh/authorized_keys
+	# workaround for shutdown race condition: http://comments.gmane.org/gmane.linux.arch.general/48739
+	/usr/bin/curl --output /etc/systemd/system/poweroff.timer https://raw.github.com/elasticdog/packer-arch/master/poweroff.timer
 
 	# clean up
 	/usr/bin/pacman -Rcns --noconfirm gptfdisk

poweroff.timer

@@ -0,0 +1,6 @@
+[Unit]
+Description=Delayed poweroff
+
+[Timer]
+OnActiveSec=1
+Unit=poweroff.target