Get timestamp from filesystem if repomd.xml cannot be parsed.

The current rpm-ostree expects the metalink to point to a binary 'summary'
file, which we are currently generating after the compose.  But, for
mirrormanager to handle a binary file instead of a repomd.xml, this workaround
is required.

https://fedorahosted.org/fedora-infrastructure/ticket/4585
Merge branch 'master' of github.com:fedora-infra/mirrormanager2