This ensures that `port_map` along with other block like attribute
declarations (e.g. ulimit, labels, etc) can handle various hcl and json
syntax that was supported in 0.8.

In 0.8.7, the following declarations are effectively equivalent:

```
// hcl block
port_map {
  http = 80
  https = 443
}

// hcl assignment
port_map = {
  http  = 80
  https = 443
}

// json single element array of map (default in API response)
{"port_map": [{"http": 80, "https": 443}]}

// json array of individual maps (supported accidentally iiuc)
{"port_map: [{"http": 80}, {"https": 443}]}
```

We achieve compatbility by using `NewAttr("...", "list(map(string))",
false)` to be serialized to a `map[string]string` wrapper, instead of using
`BlockAttrs` declaration.  The wrapper merges the list of maps
automatically, to ease driver development.

This approach is closer to how v0.8.7 implemented the fields [1][2], and
despite its verbosity, seems to perserve 0.8.7 behavior in hcl2.

This is only required for built-in types that have backward
compatibility constraints.  External drivers should use `BlockAttrs`
instead, as they see fit.

[1] https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/docker.go#L216
[2] https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/docker.go#L698-L700