Change arch platform's init style to systemd

Here's the initial work.

I tested against the images from https://github.com/elasticdog/packer-arch.  I ran the tests for fedora19 and it looks passed.
In systemd-213-5, Using variables for PIDfile doesn't seem to work.  I'm not sure what changed with the one in Fedora 19 (systemd-204-8).

Before the change, the chef-client gets killed
```
   Loaded: loaded (/etc/systemd/system/chef-client.service; enabled)
   Active: inactive (dead) since Tue 2014-06-10 13:08:38 UTC; 8s ago
  Process: 16298 ExecStart=/usr/bin/chef-client -d -c $CONFIG -P $PIDFILE -i $INTERVAL -s $SPLAY (code=exited, status=0/SUCCESS)
 Main PID: 16300 (code=exited, status=0/SUCCESS)

Jun 10 13:08:37 vagrant-arch.vagrantup.com chef-client[16298]: [2014-06-10T13:08:37+00:00] INFO: Daemonizing..
Jun 10 13:08:37 vagrant-arch.vagrantup.com chef-client[16298]: [2014-06-10T13:08:37+00:00] INFO: Forked, in 16303. Privileges: 0 0
Jun 10 13:08:38 vagrant-arch.vagrantup.com systemd[1]: Started Chef Client daemon

```

here's the message with this workaround:

```
   Loaded: loaded (/etc/systemd/system/chef-client.service; enabled)
   Active: active (running) since Tue 2014-06-10 13:17:59 UTC; 28s ago
  Process: 16856 ExecReload=/bin/kill -HUP $MAINPID (code=exited, status=0/SUCCESS)
  Process: 17350 ExecStart=/usr/bin/chef-client -d -c $CONFIG -P $PIDFILE -i $INTERVAL -s $SPLAY (code=exited, status=0/SUCCESS)
 Main PID: 17355 (chef-client)
   CGroup: /system.slice/chef-client.service
           `-17355 /usr/bin/ruby /usr/bin/chef-client -d -c /etc/chef/client.rb -P /var/run/chef/client.pid -i 1800 -s 300

Jun 10 13:17:58 vagrant-arch.vagrantup.com chef-client[17350]: [2014-06-10T13:17:58+00:00] INFO: Daemonizing..
Jun 10 13:17:58 vagrant-arch.vagrantup.com chef-client[17350]: [2014-06-10T13:17:58+00:00] INFO: Forked, in 17355. Privileges: 0 0
Jun 10 13:17:59 vagrant-arch.vagrantup.com systemd[1]: chef-client.service: Supervising process 17355 which is not our child. We'll most likely not notice when it exits.
Jun 10 13:17:59 vagrant-arch.vagrantup.com systemd[1]: Started Chef Client daemon.

```