Description of problem:
mock cannot work properly using dnf. the yum command does not work either since it is using dnf.

Version-Release number of selected component (if applicable):
$ rpm -q mock dnf yum
mock-1.2.7-1.fc22.noarch
dnf-0.6.5-1.fc22.noarch
yum-3.4.3-505.fc22.noarch

How reproducible:
Always

Steps to Reproduce:
1. Install Fedora 22 
2. Configure mock 
3. mock -r epel-7-x86_64 --init

Actual results:
dnf is trying to do weird things, like use my systems repos, require a releasever and so on:
$ mock -r epel-7-x86_64 --init
INFO: mock.py version 1.2.7 starting (python version = 3.4.2)...
Start: init plugins
INFO: selinux enabled
Finish: init plugins
Start: run
Start: clean chroot
Finish: clean chroot
Start: chroot init
INFO: calling preinit hooks
INFO: enabled root cache
INFO: enabled yum cache
Start: cleaning yum metadata
Finish: cleaning yum metadata
INFO: enabled ccache
Mock Version: 1.2.7
INFO: Mock Version: 1.2.7
Start: yum install
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/epel-7-x86_64/root/ --releasever 7 install @buildsys-build --setopt=tsflags=nocontexts'


You have enabled checking of packages via GPG keys. This is a good thing.
However, you do not have any GPG public keys installed. You need to download
the keys for packages you wish to install and install them.
You can do that by running the command:
    rpm --import public.gpg.key


Alternatively you can specify the url to the key you would like to use
for a repository in the 'gpgkey' option in a repository section and yum
will install it for you.

For more information contact your distribution or package provider.

ERROR: Command failed. See logs for output.
 # /usr/bin/yum --installroot /var/lib/mock/epel-7-x86_64/root/ --releasever 7 install @buildsys-build --setopt=tsflags=nocontexts
Problem repository: [google-talkplugin]
ip_resolve: None
proxy_password: None
deltarpm: True
bandwidth: 0
cost: 1000
exclude: []
repo_gpgcheck: False
sslverify: True
gpgkey: []
minrate: 0
fastestmirror: False
priority: 99
mirrorlist: None
gpgcheck: True
include: []
sslcacert: None
username: None
metadata_expire: 172800
sslclientcert: None
baseurl: [u'http://dl.google.com/linux/talkplugin/rpm/stable/x86_64']
proxy: None
throttle: 0
password: None
mediaid: None
name: u'google-talkplugin'
sslclientkey: None
metalink: None
enabled: True
skip_if_unavailable: True
proxy_username: None
timeout: 120
enablegroups: True

Expected results:
mock works, doesn't necessarily require a releasever, doesn't try to use my systems yum repos, etc.

Additional info:
If I add to the cfg it works as expected:
config_opts['package_manager'] = 'yum'
config_opts['yum_command'] = '/usr/bin/yum-deprecated'

It looks like yum is actually the default, but maybe making the default yum command yum-deprecated might make it work properly without editing each cfg file.