Description of problem:
When trying to run 'fedpkg mockbuild' on the master branch for a package, I get an error.

Version-Release number of selected component (if applicable):
fedpkg-1.19-2.fc22.noarch
mock-1.2.7-1.fc22.noarch
yum-3.4.3-505.fc22.noarch
dnf-0.6.5-1.fc22.noarch

How reproducible:
100%

Steps to Reproduce:
1.For any package, check out the master branch
2.fedpkg mockbuild

Actual results:
e.g.:

Wrote: /home/twaugh/devel/fedora/cups/cups-2.0.2-5.fc23.src.rpm
INFO: mock.py version 1.2.7 starting (python version = 3.4.2)...
Start: init plugins
INFO: selinux enabled
Finish: init plugins
Start: run
INFO: Start(/home/twaugh/devel/fedora/cups/cups-2.0.2-5.fc23.src.rpm)  Config(rawhide-x86_64)
Start: clean chroot
Finish: clean chroot
Start: chroot init
INFO: calling preinit hooks
Mock Version: 1.2.7
INFO: Mock Version: 1.2.7
Start: yum install
ERROR: Exception(/home/twaugh/devel/fedora/cups/cups-2.0.2-5.fc23.src.rpm) Config(rawhide-x86_64) 0 minutes 0 seconds
INFO: Results and/or logs in: /home/twaugh/devel/fedora/cups/results_cups/2.0.2/5.fc23
INFO: Cleaning up build root ('cleanup_on_failure=True')
Start: clean chroot
Finish: clean chroot
ERROR: Command failed. See logs for output.
 # /usr/bin/yum --installroot /var/lib/mock/rawhide-x86_64/root/ groupinstall build --setopt=tsflags=nocontexts
Yum command has been deprecated, use dnf instead.
See 'man dnf' and 'man yum2dnf' for more information.
To transfer transaction metadata from yum to DNF, run 'dnf migrate'Redirecting to '/usr/bin/dnf --installroot /var/lib/mock/rawhide-x86_64/root/ groupinstall build --setopt=tsflags=nocontexts'

Config error: releasever not given and can not be detected from the installroot.
Could not run mockbuild: Command '['mock', '--configdir', '/tmp/fedora-devel-x86_64.Vvshrbmockconfig', '-r', 'fedora-devel-x86_64', '--resultdir', '/home/twaugh/devel/fedora/cups/results_cups/2.0.2/5.fc23', '--rebuild', '/home/twaugh/devel/fedora/cups/cups-2.0.2-5.fc23.src.rpm']' returned non-zero exit status 1

Expected results:
Build root set up and mock build proceeds.

This is different from bug #1210398.