docker: Generate /packages.txt in centos6 image

Put the list of package names in an environment, and output their
package names to the target file in the end.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <1474429768-25027-2-git-send-email-famz@redhat.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>

tests/docker/dockerfiles/centos6.docker

 FROM centos:6
-RUN yum install -y \
+RUN yum install -y epel-release
+ENV PACKAGES libfdt-devel ccache \
     tar git make gcc g++ \
     zlib-devel glib2-devel SDL-devel pixman-devel \
     epel-release
-RUN yum install -y libfdt-devel ccache
+RUN yum install -y $PACKAGES
+RUN rpm -q $PACKAGES | sort > /packages.txt