[Yum] support downgrading off packages

[seth vidal]

> At the moment, this is just considered one of the things you accept
> when you run rawhide.  I agree that it would be nice, but it's almost
> never an issue except when people decide to do fairly crazy things
> (like run rawhide).  Therefore, I think it's not a high priority.
> 

I am probably of the opinion that it will never happen. It MIGHT be
possible eventually to say 'I want to go to THIS VERSION' and it will
load that version, provided it can get the dependencies for it.

but I don't think I'll implement yum downgrade yourmom anytime soon.

-sv