check_software: check egg vulnerabilities for every used python versions

pkg_resources.Environment only scans packages for the current python
version, but the checked software might be using a different python
version than the python version used to check software (the slapos
python). The checked software might also include packages for
multiples python versions.

Compute the list used python packages from the eggs directory and use
an Environment for each python version, this way we check all python
versions and also when slapos python and software python are different.