Addresses #998 to properly assign a random port and access the port assigned

[shellscape]

What kind of change does this PR introduce?
Feature and Fix

Did you add or update the examples/?
No need

Summary
#998 requested the use of random ports and the ability to access the assigned random port.

Does this PR introduce a breaking change?
It introduces a corrective change. This PR also corrects the port reported by the CLI when a random port is assigned.

Other information

To access the port assigned you may go off of this example, or the following sample code:

const Server = require("webpack-dev-server/lib/Server");
conast server = new Server(compiler, options);

server.listen(0, '127.0.0.1', () => {
  const port = server.listeningApp.address().port;
});

[codecov]

Codecov Report

Merging #1054 into master will decrease coverage by 0.16%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #1054      +/-   ##
==========================================
- Coverage   73.54%   73.37%   -0.17%     
==========================================
  Files           5        5              
  Lines         446      447       +1     
  Branches      140      141       +1     
==========================================
  Hits          328      328              
- Misses        118      119       +1

Impacted Files	Coverage Δ
lib/util/createDomain.js	37.5% <25%> (-5.36%)	⬇️

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