consul tls ca create ignores filename-prefix

Hi Team,

I may just be interpreting the documentation incorrectly, apologies if that's the case.
The documentation for the `consul tls` helper function for creating consul pki certificates implies that a filename prefix can be added as follows:

` Usage: consul tls ca create [filename-prefix] [options]` [source](https://www.consul.io/docs/commands/tls/ca.html)

However when I try using a filename prefix of bananas, it's ignored:

``` bash
sudo /usr/local/bin/consul tls ca create bananas
==> Saved consul-agent-ca.pem
==> Saved consul-agent-ca-key.pem
```

Looking at the code it appears the the filename prefix is actually taken by reading the `-domain` flag [source](https://github.com/hashicorp/consul/blob/master/command/tls/ca/create/tls_ca_create.go#L53-54)

And by testing I get the desired result:

``` bash
sudo /usr/local/bin/consul tls ca create -domain=bananas
==> Saved bananas-agent-ca.pem
==> Saved bananas-agent-ca-key.pem
```

If the code is correct I think that it may be clearer to remove the ` [filename-prefix] ` from the usage examples and just update the options section for `-domain` to mention this is also used as the prefix for the generated files?

Thank you.