automation: Ensure that all packages are up-to-dates in CentOS 7 container

This was prompt by the fact that `wget https://xenbits.xenproject.org`
fails with expired certificates, which turned out to be an expired
root certificates. Updating all packages fix the issue.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

automation/build/centos/7.dockerfile

@@ -15,7 +15,8 @@ RUN rpm --rebuilddb && \
     rm -rf /var/cache/yum
 
 # install Xen depends
-RUN yum -y install \
+RUN yum -y update \
+    && yum -y install \
         gcc \
         gcc-c++ \
         ncurses-devel \