Remove previous yum.repos.d to only used the custom ones

There are some repos that are deactivated in the hosts
but not deactivated in the docker images, like
CentOS-QEMU-EV.repo, this patch remove the old repos dir
to just use the the ones injected.

Change-Id: I15284189fda16514f7ba70b95ab551d58eca7ab1
Closes-Bug: 1779642

templates/Dockerfile.j2

@@ -7,6 +7,7 @@ COPY yum_update.sh /tmp/
 COPY compare-package-json.py /tmp/
 
 {% if yum_repos_dir_path is defined %}
+RUN rm -rf /etc/yum.repos.d/
 COPY yum.repos.d /etc/
 COPY repos /
 {% endif %}