dnf: default to disable systemd units

The services and timers are not useful if a repo is not configured.
In OE, we default to have no repo configured. So we default to disable
these units.

The problem was found by the test_systemd_failed runtime test case.
dnf-makecache.service is found to fail. This is due to lack of repo.
Note that as this service is started by the dnf-makecache.timer, it's
possible that test_systemd_failed passes in case the timer is not started
yet.

(From OE-Core rev: c017f5528d1ae6316c9434ffd8702665bfe069f9)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/dnf/dnf_2.7.5.bb

@@ -72,3 +72,4 @@ SYSTEMD_SERVICE_${PN} = "dnf-makecache.service dnf-makecache.timer \
                          dnf-automatic-install.service dnf-automatic-install.timer \
                          dnf-automatic-notifyonly.service dnf-automatic-notifyonly.timer \
 "
+SYSTEMD_AUTO_ENABLE ?= "disable"