add retain_package_versions_count to repository-module

[bastolino]

SUMMARY

When setting the mirroring policy to additve, there is another configuration field showing up in foreman called "retain package versions". This field is not yet configurable via ansible.
Basically it's just another field in the json object when calling the api on /katello/api/v2/repositories/?organization_id= called "retain_package_versions_count", type int on same hierarchy level as mirroring_policy. This must only be set when mirroring_policy is set to "additive", disregard else.

ISSUE TYPE

• Feature Idea