Change jobspec for `job init` command

### Proposal

The jobspec returned by `nomad job init` should be updated.

Currently it returns a Redis job. This is confusing in that Redis is a stateful job, but the init job doesn't have any volume support.

### Use-cases

* Show off interesting Nomad features
* Provide a better "Aha moment" to new Nomad users
* Avoid confusing the user with stateful job immediately.

### A proposed Jobspec

This could go in a lot of different directions, but here is a proposal to get the ball rolling.

* Job has two tasks. - Demonstrates task hierarchy.
* One task is running Docker, the other is running exec. - Demonstrates driver flexibility.
* One job is making a request to the other using Nomad Service Discovery - Demonstrates simple service discovery for new users.
* The "inner" task is using the template stanza - Demonstrates templating well. We could inject a Secure Variable here when we ship these.
* The "inner" task has a count of more than 1. This would help show off load balancing within Nomad and multiple allocs would help people understand Nomad's model better.
* The outer task could be Nginx running docker potentially? Or maybe just something that makes a request to the inner task and wraps it in a pretty UI.
* Ideally the UI that is returned has an explanation of what is happening (so they user can hit a URL and get more info), and links to next steps, such as the Nomad learn guides.
* Neither task should be stateful, as this is confusing for new users. Perhaps volume docs can be linked to from the UI.

Start with the `nomad job init` command and inspect how its current Redis jobspec is assembled. The issue proposes several possible demonstrations, so completion would require settling the example's scope and replacing the stateful jobspec with a clear, non-stateful one.

Written by the indexing model from the issue text.