Add a new stage for modifying YUM global configuration.
Add a unit test case for the newly added stage.
Because we test stages on Fedora, where there is no YUM, and this stage
is mostly intended for being used with RHEL-7 images, the stage does not
produce error in case the `/etc/yum.conf` file does not exist. It rather
produces a warning and creates the file. Ideally the stage would produce
an error in case the configuration file does not exist, but that would
be impossible to test on recent Fedora.
Signed-off-by: Tomas Hozza <thozza@redhat.com>
58ec1c9a83