Description of problem:
Currently copr supports "$releasever" and "$basearch" in repo urls for
building packages, but "fedora-$releasever-$basearch" only works for
Fedora not EPEL.
If "$osrelease" was "fedora" on Fedora and "epel" on EPEL" then repo urls
for build dep repos could use "$osrelease-$releasever-$basearch" to build
for both fedora and epel.

Steps to Reproduce:
1. Add http://copr-be.cloud.fedoraproject.org/results/USER/REPO/fedora-$releasever-$basearch/ repo url to a COPR repo config.
2. Configure repo for EPEL builds.
3. Build repo

Actual results:
Build fails on EPEL due to missing dep since no fedora-<epel-ver>-arch repo!

Expected results:
Some way to handle both fedora and epel copr dep repos.