namespace not being created in remote region

### Nomad version
Nomad v1.1.0-beta1 (099d2c222208b04878a5528143da0a80b9666f0a)

### Operating system and Environment details

A multi-region deployment: 2 or more clusters

### Issue

Creating a namespace in a multi-region deployment is only working in the authoritative region.

According to the [source code](https://github.com/hashicorp/nomad/blob/87af5dec787cd933b4ac8d22ac872f35408e266d/nomad/namespace_endpoint.go#L22), namespace creation is always forwarded to the Authoritative region... Are they supposed to be forwarded and synchronized across all regions?

### Reproduction steps

1. List namespace in region A:
```
$ nomad namespace list -region=A
Name Description
default Default shared namespace
```
2. Add namespace in region A:
```
$ nomad namespace apply -region=A ns-alpha
Successfully applied namespace "ns-alpha"
```
3. List namespace in region A:
```$ nomad namespace list -region=A
Name Description
default Default shared namespace
ns-alpha
```
4. List namespace in region B:
```
$ nomad namespace list -region=B
Name Description
default Default shared namespace
```
5. Add namespace in region B:
```
$ nomad namespace apply -region=B ns-beta
Successfully applied namespace "ns-beta"
```
6. List namespace in region B:
```$ nomad namespace list -region=B
Name Description
default Default shared namespace
```

#### Expected Result

```$ nomad namespace list -region=B
Name Description
default Default shared namespace
ns-beta
```

#### Actual Result

No namespace created in region B.