Force product-id yum plugin to respect the managed root

Originally, the plugin updated product certificates of the host even if the
Yum's root was different. That meant that host certificates were matched
against RPMDB of the chroot. In the end, it could damage the host system.
Force subscription-manager yum plugin to respect the managed root

Originally, the plugin updated the repo file of the host even if the Yum's
root was different. It did not make any sense. If a chroot is managed,
user expects to work with actual repos there.

To obtain Yum's root, we need to wait until it's configuration is processed.
Thus we have to hook to the next slot. As long as we modify only repo files,
it's ok.