register: false doesn't disable the registration

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
I set `register: false` in the NbPasswordAuthStrategy. but the registration link is still open for the user to click.

**Expected behavior:**
If the `register` is set to false, then the registration link should be hidden.

### Other information:

**npm, node, OS, Browser**
```
mac High Sierra, node 8.11.1, Chrome
```

**Angular, Nebular**
```
angular 6.0.0, nebular 2.0.0-rc.9
```