Description of problem:
Customers using Satellite prior the introduction of On-Demand policy for repos are consuming a *lot* of space in mirroring not only metadata but also binary rpms on their satellites.

Migrating all the existing repos to On-Demand policy do not affect the previously downloaded content.

In order to get rid of the previously downloaded content, atm is needed to remove the repo synced into sat6. But this require to delete all the CVs making use of that repo.

Version-Release number of selected component (if applicable):
sat6.2.latest
sat6.3

How reproducible:
always

Steps to Reproduce:
1. sync a repo with immediate policy
2. (/var/lib/pulp/content/units/rpm start growning)
2. change the policy to on-demand

Actual results:
/var/lib/pulp/content/units/rpm stop growning but do not delete content not really needed anymore

Expected results:
free up space in: /var/lib/pulp/content/units/rpm

If all the needed bits are in place, this could be just a doc bug, if there are tech elements to be implemented, let's do it: that's the goal