[feature]/[question] add operators to distinct_hosts (specify minimum number of distinct_hosts required)

I often find myself wishing nomad was able to schedule job in a way that places groups on at least two or three different hosts. If service count is low (think 2 or 3) I can get away with using `distinct_hosts` but for high count groups this becomes unusable very fast as number of not fully packed hosts rises over time.

I don't know how feasible it is, but something like this would be extremely useful and a default for me even for stateless services:

```
job "service" {
group "example" {
count = 10
constraint {
operator = "distinct_hosts"
operator = ">="
value = "2"
}
...
}
}
```

EDIT: Please note that this is different and probably conflicting option to the one described here https://github.com/hashicorp/nomad/issues/1387 :)

Start by comparing the requested distinct_hosts minimum-count behavior with the conflicting proposal in issue 1387. Then trace the existing distinct_hosts constraint from the job configuration example into Nomad's scheduling implementation and tests; done means an agreed operator syntax and defined placement behavior for high-count groups.

Written by the indexing model from the issue text.