Make platform yum-registry disabled by default

Recently we deleted the nexus persistance volume claim in order to try to upgrade nexus. This has deleted all of our rpms and has made the yum registry unusable.

Since then, we're unable to re-build some of our repository because they need to access the yum registry.

This is the case for the builder-rpm which allows to build nuxeo-rpms which allows to make yum registry usable.

In the end, we want to disable by default the nuxeo repository and enable it on the command line when needed.