validate repomd.xml files

[jonathanspw]

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 These files are integral to a working mirror system even if other packages are out of date, missing, etc.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.

[jonathanspw]

This cropped up again in #959 . We should prioritize this work soon.