[PYTHON] use customized default Configuration() objects if available

Created by: adpoliak

To be reviewed by the Python Technical Committee:
@taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @arun-nalla @spacether

For the Python variants, the generated PACKAGENAME.configuration.Configuration class allows the programmer to save the data from pre-configured instance of it as the default to be used for future instances. However, this requires the classes to be instantiated using Configuration.get_default_copy(), rather than just Configuration(). This PR changes the templates/samples to use the Configuration.get_default_copy() wherever a naked Configuration() was found. This has the effect of making the programmer's stored Configuration be used (such as to override the client-side verification setting), if the programmer set one, and the hard-coded defaults used otherwise.

Related Issue: #8499

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.