#450 - allow configuration of manifest dir

Add the possibility to configure the `manifest_dir` as with Vagrant. Unfortunately I can't get it to work:

```
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-provisioner-puppet-masterless: 2013/11/25 09:19:52 Preparing to serve a provisioner plugin...
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-provisioner-puppet-masterless: 2013/11/25 09:19:52 Plugin build against Packer '8a08a00d820eac77db61229bfdf08aeabb944408+CHANGES'
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-provisioner-puppet-masterless: 2013/11/25 09:19:52 Plugin minimum port: 10000
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-provisioner-puppet-masterless: 2013/11/25 09:19:52 Plugin maximum port: 25000
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-provisioner-puppet-masterless: 2013/11/25 09:19:52 Plugin address: 127.0.0.1:10008
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-provisioner-puppet-masterless: 2013/11/25 09:19:52 Waiting for connection...
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-provisioner-puppet-masterless: 2013/11/25 09:19:52 Serving a plugin connection...
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-command-build: 2013/11/25 09:19:52 Skipping build 'vmware' because not specified by -only.
virtualbox output will be in this color.
2013/11/25 09:19:52 ui: virtualbox output will be in this color.

2013/11/25 09:19:52 ui:
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-command-build: 2013/11/25 09:19:52 Build debug mode: false
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-command-build: 2013/11/25 09:19:52 Force build: false
2013/11/25 09:19:52 /Users/jan/Projects/bin/packer-command-build: 2013/11/25 09:19:52 Preparing build: virtualbox
1 error(s) occurred:

* Unknown configuration key: manifest_dir
2013/11/25 09:19:52 ui error: 1 error(s) occurred:

* Unknown configuration key: manifest_dir
```

Packer complains that `manifest_dir` is unknown, yet the test passes.