FIXME: cobbler: setup.sh: Keep extra RPM repos.

FIXME: cobbler: setup.sh: Keep extra RPM repos.

cobbler-grub-aarch64 (arm64 specific loader) is currently
distributed and added to the cobbler docker container during
ISO build using EXTRA_RPM_REPOS envvar mechanism.

However, since this package installs files in a mounted location,
its contents are not persistent, requiring a test and eventual
forced reinstall during container start.

This, of course, requires the package to be available at that point,
which is not currently possible, as we remove the extra RPM repository
entry from yum config at the end of container build.

Hacky temporary workaround:
Keep extra RPM yum config after container is set up, only for cobbler.

TODO: This patch should be dropped once cobbler-grub-aarch64 RPM
package gets upstreamed.

Change-Id: I58e39df2671d79125c68a5ec994db962c103ce01