Run yum clean to reduce size of docker image layer

This ensures we do not write the yum cache in the layer, making the
image size smaller.

Change-Id: I974bcac75be9231cf282a73db1a22a6f4ba079fc
(cherry picked from commit 0c2305ad80)

container-images/tripleo_kolla_template_overrides.j2

@@ -453,5 +453,5 @@ STOPSIGNAL SIGTERM
 # a yum update in the kolla base image. All the other images should inherit this
 # but if the base distro container is out of date (i.g. 7.4 but 7.5 is out) this
 # will pull in the updated packages available. Related issue LP#1770355
-RUN yum update -y
+RUN yum update -y && yum clean all
 {% endblock %}