RPM Support

Ticket moved to GitHub: "pulp/pulp_rpm/2282":https://github.com/pulp/pulp_rpm/issues/2282

This issue is best understood by comparing retain_package_versions with the similar retained_versions options from pulpcore: retained_versions uses Integer >= 1 plus Null value, where retain_package_versions uses integer >= 0 and no Null value. Most pulp options use Null to mean "ignore the effect of this option completely". In addition "retain 0 packages of a version" = "retain all versions of the package" is inherently unintuitive. Both for consistency with the rest of pulp and the "natural language" meaning, retain_packgae_versions should use Null to mean "Ignore this option/keep all versions".

See the community discussion here: https://github.com/pulp/community/discussions/36

P.S.: I am also wondering if the retain_package_versions makes sense for other plugins that host "packages", like python and Debian. In that case it might be good to move this feature to pulpcore. (As far as I can tell the implementation uses age and is not RPM specific). But I will open a separate issue for that.