[Feedback] Nomad 0.11 lifecycle insufficient for 3 phase deploys

Nomad 0.11 introduced `lifecycle` stanza which is a much needed resource. According to the [blogpost](https://www.hashicorp.com/blog/hashicorp-nomad-task-dependencies/) you are looking for feedback

One use case this doeesn't cover is when you have 3 steps

- postgres server
- seed database
- start web server

Here is an example job

https://gist.github.com/spuder/54f4b54a86d4690a76bfc4ed8eecc415

I've set the following

- postgres server
```
task "postgres" {
driver = "docker"
config {
image = "postgres:9.6-alpine"
}
lifecycle {
sidecar = true # <= calling this a sidecar doesn't really seem right. It does ensure it will keep running
hook = "prestart"
}
```
- seed database
```
task "seed_database" {
driver = "docker"
lifecycle {
sidecar = false # <= this allows the job to run then exit
hook = "prestart"
}
config {
image = "redash/redash:8.0.0.b32245"
command = "create_db"
}
...
```
- start web server

While this _mostly_ works, and the seed database job always runs before the web server, it doesn't guarantee that postgres will be up before the seed database job.

There needs to be a way to allow for 3 services to be chained together

Start with Nomad's lifecycle stanza and the linked example job, focusing on how the postgres, seed_database, and web server tasks are ordered. Done means a three-phase deployment can guarantee that postgres is ready before the seed job runs, and that the web server starts afterward.

Written by the indexing model from the issue text.