Caches aren't invalidated if you switch instrepo / version

[wgwoods]

If you do something like fedup --network 19, and then decide, whoops, I want to go to 20, we'll still use the cached metadata from the Fedora 19 versions of all the repos.

We need to invalidate the cached metadata whenever the user changes the version or the URL of --instrepo.

[wgwoods]

We could probably fix most of this by using the default yum metadata dir for our repos. This would also mean that the system doesn't need to redownload the new repodata after the upgrade, which would be kind of helpful.

--instrepo is a little different, but saving and examining the fedup arguments also relates to #51, where we'd want to save sys.argv or the args object so we can implement fedup --continue..