pause_before is a valid config setting

Without this change the new pause_before setting fails config validation causing Packer to error out.

```
2013/12/27 08:47:18 ui: 
2013/12/27 08:47:18 /Users/sneal/src/packer/bin/packer-command-build: 2013/12/27 08:47:18 Build debug mode: false
2013/12/27 08:47:18 /Users/sneal/src/packer/bin/packer-command-build: 2013/12/27 08:47:18 Force build: false
2013/12/27 08:47:18 /Users/sneal/src/packer/bin/packer-command-build: 2013/12/27 08:47:18 Preparing build: vmware-iso
1 error(s) occurred:

* Unknown configuration key: pause_before
2013/12/27 08:47:18 ui error: 1 error(s) occurred:

* Unknown configuration key: pause_before
```