This PR enhances the distinct_property constraint such that a limit can
be specified in the RTarget/value parameter. This allows constraints
such as:
```
constraint {
distinct_property = "${meta.rack}"
value = "2"
}
```
This restricts any given rack from running more than 2 allocations from
the task group.
Fixes https://github.com/hashicorp/nomad/issues/1146
Caught some typos. Made units separate from the numbers 1GHz -> 1 GHz
after talking to Nick about questions of style (this has the side effect of making future spell checking easier).
