Run yum clean all in Heka Dockerfile

This follows up on a review comment from Steven Dake.  See
https://review.openstack.org/#/c/275840/14/docker/heka/Dockerfile.j2@21

Closes-Bug: #1547535
Partially implements: blueprint heka

Change-Id: I4f5094145d8f5bbc04fe23e64c6a39719775bae4

docker/heka/Dockerfile.j2

@@ -4,6 +4,7 @@ MAINTAINER {{ maintainer }}
 {% if base_distro in ['centos', 'fedora', 'oraclelinux', 'rhel'] %}
 
 RUN yum -y install https://github.com/mozilla-services/heka/releases/download/v0.10.0/heka-0_10_0-linux-amd64.rpm \
+    && yum clean all \
     && useradd --user-group heka
 
 {% elif base_distro in ['ubuntu'] %}