Update live image build for a change to upstream mock

https://github.com/rpm-software-management/mock/commit/cb25d3ba
changes existing mock configs for stable releases to have a
`dnf.conf` section instead of a `yum.conf` section, and this
change got pushed out to F30 and F31, which breaks us :( Our
mock config that we use for building live images assumes the
existence of a `yum.conf` section in the config it inherits
from.

This change is now stable for F30 and F31, so at least we don't
have to do any conditional shenanigans; we can just change to
'the new style' unconditionally and things should work OK.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

tests/_live_build.pm

@@ -19,7 +19,7 @@ sub run {
     assert_script_run 'echo "config_opts[\'plugin_conf\'][\'bind_mount_opts\'][\'dirs\'].append((\'/opt/update_repo\', \'/opt/update_repo\'))" >> /etc/mock/openqa.cfg';
     assert_script_run 'echo "config_opts[\'plugin_conf\'][\'bind_mount_opts\'][\'dirs\'].append((\'/dev/' . $serialdev . '\', \'/dev/' . $serialdev . '\'))" >> /etc/mock/openqa.cfg';
     # add the side repo to the config
-    assert_script_run 'printf "config_opts[\'yum.conf\'] += \"\"\"\n[advisory]\nname=Advisory repo\nbaseurl=file:///opt/update_repo\nenabled=1\nmetadata_expire=3600\ngpgcheck=0\n\"\"\"" >> /etc/mock/openqa.cfg';
+    assert_script_run 'printf "config_opts[\'dnf.conf\'] += \"\"\"\n[advisory]\nname=Advisory repo\nbaseurl=file:///opt/update_repo\nenabled=1\nmetadata_expire=3600\ngpgcheck=0\n\"\"\"" >> /etc/mock/openqa.cfg';
     # replace metalink with mirrorlist so we don't get slow mirrors
     repos_mirrorlist("/etc/mock/openqa.cfg");
     # upload the config so we can check it's OK