Add a "core" sub-command to jenkins-plugin-manager CLI

The jenkins-plugin-manager CLI should be able to download the latest jenkins package (default debian-stable) listed in http://updates.jenkins.io/stable/latestCore.txt.

Currently the jenkins_plugin_manager.core module supports already the download, this just need to be called correctly.

  jenkins-plugin-manager core download [--distribution <debian|debian-stable>] [--version <jenkins core version>] [--repository <https://pkg.jenkins.io>]

This will fetch the latest (or specified) jenkins package.
The package checksum should be checked from https://pkg.jenkins.io/debian-stable/binary/Packages

Alternatively, python-apt could be leveraged to configure temporarily and download the package, providing an automatic checksum.