Always use configured host at runtime

[philip]

• If host is defined/configured at runtime, then set host to the value set
  at runtime, otherwise use the default
• Currently the runtime value is ignored unless you do something like
  the following (instantiate the api client, empty or otherwise):
  $api_client = new Swagger\Client\ApiClient();
  $foo = new Swagger\Client\Api\Bar($api_client);
  But now all you need is:
  $foo = new Swagger\Client\Api\Bar();
• This use case is common if host is not defined in the swagger file
• This now allows the host to be set at runtime under any condition,
  for example using either:
  • lib/Configuration.php $host
  • Swagger\Client\Configuration::getDefaultConfiguration()->setHost($host)
• This could change behavior for some, as it means the configured host
  is used instead of the default, unless api client is instantiated first.
  Not sure why someone would do that, likely by accident.
• This resolves [PHP] lib/Api constructors ignore configuration and set host to localhost  #4518

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

[philip]

This was fixed in PR #4525 so closing this PR now.