Force yum plugins to respect the managed root

This is likely everything what I need for the Mock integration. I mean the new ```RepoUpdateActionCommand``` and ```subscription_manager.utils.chroot```. I'll try to call the API from Mock. If that approach won't be accepted (because Mock calls every tool via CLI so far), I'll push appropriate entry point. On the other hand if you already expected that entry point from me, I can push it immediately.

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.