[Feature Request] Property to set prestart task to run only one instance even if in larger group

It would be great to be able to set a property for a prestart task to run only as a single instance even if the task group has a count larger than 1.

The use case for this is that we are using prestart tasks to populate Vault secrets, and if the group has a count larger than 1 then the prestart task will also be executed multiple times, resulting in multiple versions of the same secrets being updated in the vault secret kv in our case.

So something like the below is what I'm proposing, I redacted all other job stanzas to keep the example clear. The lifecycle "mode" property could consist of options "single" and "group" to begin with, defaulting to "group", meaning it will start up as many instances as the group count declares.

```
group "example" {
count = 3

task "prestart" {
lifecycle {
hook = "prestart"
mode = "single"
}

driver = "docker"

config {
image = "alpine:3.11"
entrypoint = ["myPrestart.sh"]
}
}

task "main" {
driver = "docker"

config {
image = "myImage"
}
}
```

Start by tracing Nomad's prestart lifecycle and how task-group counts launch hook tasks. Define and test the proposed single-versus-group behavior, including the default, with completion shown by a prestart task running once when the group count is three.

Written by the indexing model from the issue text.