Add missing 'yum clean all' to mariadb

Found another missing clean in mariadb, halves the size of the image.

TrivalFix

Change-Id: Idcbb925a6f232bd1172710456662a52784e025b6

docker/mariadb/Dockerfile.j2

@@ -14,6 +14,7 @@ RUN yum install -y \
         pv \
         tar \
         expect \
+    && yum clean all \
     && rm -rf /var/lib/mysql/*
 
 {% elif base_distro in ['ubuntu', 'debian'] %}