Bug 1053940 - docker-worker: Encrypted environment variables

One open issue is the management of the gpg keys. Currently they are hard-coded.
We should not add `created` and `deadline` to `task.payload`...
We already have a `created` and a `deadline` in `task` as `task.created` and `task.deadline`.

But after further though I think you can remove this check, as well as the check:
`if (encryptedData.deadline !==  payload.deadline) {`

Basically, I think it's an unnecessary assertions. And more importantly, I fear that requiring the same `created` and `deadline` timestamps as is set on `task`, will make encryption of environment variables more complicated. Especially, if we do the encryption inside a template... I could certainly imagine template functions for issuing temporary credentials that can then be embedded using encrypted environment variables.

Hence, the checks `encryptedData.created > Date.now()` and `Date.now() > encryptedData.deadline` should be sufficient. Let me know if you have concerns about this minor suggestion. I think only checking against `Date.now()` will make it easier to create temporary credentials.