added the ability to use Debian based distributions by adding the apt-get package manager

[stephengaito]

This pull request provides changes to (the development branch of) CEKit to allow the use of apt-get as a package manager.

These changes are based upon the corresponding pull request for the Alpine apk package manager #612 corresponding to the enhancement issue #609

Following the example of the PR #612 we have added:

1. changes to the templating system (via template_helper.py),
2. updated the documentation,
3. added (minimal?) tests corresponding to the use of debian-slim and apt-get as the distribution and its package manager.

NOTE: The Debian apt-get package manager seems to require an "update" step before its "install" step. To be completely "correct" we should refactor the repo_install and pkg_install macros in the cekit/templates/template.jinja file to add an explicit "pre-install" step. However we have, for this PR, simply added update && apt-get --no-install-recommends to the package_manager_flags macro in the cekit/template_helper.py file. (see the PR differences report).

All tests pass against the Python 3.8 interpreter. (make test).

Fixes #693

[rnc]

Thanks for the contribution!