keep backwards compatible support for python-virtualenv

[oniGino]

• Puppet: 6.19.1
• Ruby:
• Distribution: Centos 7
• Module version: 6.0.0

Currently python::install will install python2-virtualenv for version 'system' on older centos machines (we run 5,6,7,8)
however
python::pyvenv does not actually support virtualenv
Please consider something like the follow added to python::pyvenv

            if versioncmp($normalized_python_version, '2.7') <= 0  {
              $virtualenv_cmd = "${python::exec_prefix}virtualenv-${normalized_python_version}"
            }

I know python 2 is very ancient but It is still widely used, and sadly I must maintain it even if I'd rather not