Force yum plugins to respect the managed root

This is probably everything what I need for the Mock integration.

This set basically forces Yum plugins to operate with chroot files (if 'yum --installroot' is used). Because Anaconda uses '--installroot' too, the chroot function can be more generic. I have not bothered with changing roots of rhsm.conf and rhsm.log, because some significant refactorings would be needed. Thus the plugins still uses the host config and log. For the case of Mock and Anaconda, it's not a problem so far. However we can expect that such a request may occur in the future.

Please note also that the Yum plugin became hooked to a later slot.

I'm ready to fix any mistake you find.