validate repomd.xml files

We should validate the content of `repomd.xml` files. These files are integral to a working mirror system even if other packages are out of date, missing, etc. the proper `repomd.xml` file will at least let `dnf` fall back to a working mirror.

Last night we had an instance of a mirror having an incorrect `repomd.xml` file resulting in a user not being able to update. We could've easily prevented this situation if we validated all `repomd.xml` files on a given mirror before considering it as healthy/online.

Marking this as a bug since in the current state non-working mirrors can be marked as OK rendering `dnf` transactions impossible without user intervention.