Change to Directory

Hi,
Trying to use "Change to Directory" (see attached screen shot)
I put in the directory the json file and an shell script.
The sh is referred in the json relatively.
I'm getting the following error

```
14:39:56 FATAL: Execution failed: /home/ubuntu/tools/biz.neustar.jenkins.plugins.packer.PackerInstallation/packer_0.9/packer build -var vpc_id=vpc-*** -var subnet_id=subnet-*** -debug /home/ubuntu/workspace/CreativeAdminBackend-BakeAMI-plugin/app-ami-packer.json
```

in the jenkins log I can see

```
launch: /home/ubuntu/tools/biz.neustar.jenkins.plugins.packer.PackerInstallation/packer_0.9/packer build -var vpc_id=*** -var subnet_id=subnet-*** -debug /home/ubuntu/workspace/CreativeAdminBackend-BakeAMI-plugin/app-ami-packer.json
java.io.IOException: Cannot run program "/home/ubuntu/tools/biz.neustar.jenkins.plugins.packer.PackerInstallation/packer_0.9/packer" (in directory "deployment"): error=2, No such file or directory
```

the folder which the json file is being looked for is the workspce dir.
*The method PackerPublisher.workingDir is returning the name of the ChangeDir and not the full path of it.
which means that the "Change to Directory" is not actually working or I don't understand its meaning.


Regards, Avihay
(Thank you for the plugin)