Bug 1053940 - docker-worker: Encrypted environment variables

One open issue is the management of the gpg keys. Currently they are hard-coded.
You can probably put it in `runtime` or something... @gregarndt may have a better overview here... It's been a while since I played with docker-worker.

But why not load `PrivateVariable` which only holds the private key... In the initializer and store an instance of `PrivateVariable` on `this.runtime`...Afaik it has no side effects...

We could probably also call it something else like...? `EnvironmentVariableDecryptor` or insert better name, I'm open to it :)   or just `PrivateKey` might be a good name. That is what it holds... Right now we only use it for decrypting env vars... but I want to decrypt ec2 user-data too, when aws-provisioner is ready to encrypt user-data :)