Make webmock allow requests to webpack too

What does this MR do?

I am running webpack on gdk.test, which I've set to 127.1.1.1 in my /etc/hosts. It's a local loopback interface, similar to (but separate from) 127.0.0.1. WebMock is already configured to allow connections to localhost (which equals to 127.0.0.1), but not outside that. So when I was running specs, I got an error from WebMock the connection was not permitted:

     WebMock::NetConnectNotAllowedError:
       Real HTTP connections are disabled. Unregistered request: GET http://gdk.test:3808/assets/webpack/manifest.json with headers {'Accept'=>'*/*', 'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3', 'User-Agent'=>'Ruby'}
     
       You can stub this request with the following snippet:
     
       stub_request(:get, "http://gdk.test:3808/assets/webpack/manifest.json").
         with(
           headers: {
              'Accept'=>'*/*',
              'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3',
              'User-Agent'=>'Ruby'
           }).
         to_return(status: 200, body: "", headers: {})
     
       ============================================================

This MR makes sure the connection is permitted.

For this to work, you'll need in your config/gitlab.yml:

test:
  webpack:
    dev_server:
      enabled: true
      host: gdk.test
      port: 3808

I'm working on a change in GDK to provide this: gitlab-development-kit!787 (merged). But if the configuration is not provided, things will remain working as before.