Bug 1053940 - docker-worker: Encrypted environment variables

One open issue is the management of the gpg keys. Currently they are hard-coded.
deploying an ami gives a permission error here (probably can't directly copy a file using packer to /etc).  
Perhaps we could do something like:
```json     
{
      "type":           "file",
      "source":         "{{user `privateKeyLocation`}}",
      "destination":    "/tmp/docker-worker-priv.pem"
    },
    {
      "type":           "shell",
      "inline":         ["sudo mv /tmp/docker-worker-priv.pem /etc/"]
    }
```