common: use EPEL metalink

[ktdreyer]

Some mirrors are stale (https://pagure.io/fedora-infrastructure/issue/11233)

Use MirrorManager's metalink application so we always get up-to-date mirrors.

MirrorManager will also return the list of mirror that carry each architecture (x86_64, aarch64, etc) so we will not need to manage that information ourselves here (like #548).

[ktdreyer]

This reverts the decision to use local mirrors, #142

@dmick mentioned that we've had issues with Fedora's mirrorlist configuration in the past. I think that removing baseurl (#544) would make this more resilient

[ktdreyer]

(also nice to stop using only plaintext http:// URLs, especially since we have gpgcheck: 0 at the moment)

[ktdreyer]

Our static mirrorlist file has download-ib01.fedoraproject.org as the first item in the list. Ordinarily that server would be stable, since the Fedora admins use it as a primary mirror, but as https://pagure.io/fedora-infrastructure/issue/11233 shows, that ib01 is now having trouble.

MirrorManager does not return this problematic mirror:

curl 'https://mirrors.fedoraproject.org/metalink?repo=epel-8&arch=x86_64' | grep ib01

[ktdreyer]

This /metalink API endpoint not only returns the list of live mirrors, but it also advertises the sha checksums of the authoritative repomd.xml file to obtain from mirrors.