Update Fedora and CentOS docker images.

- Reduce image size by skipping weak package references.
- Consolidate and sort packages to install.
- Improve consistency between versions.
- Combine yum/dnf install and clean to avoid unwanted caching.
- Don't update existing packages from base image unnecessarily.

test/utils/docker/centos7/Dockerfile

@@ -1,6 +1,5 @@
-# Latest version of centos
 FROM centos:centos7
-RUN yum -y update; yum clean all; yum -y swap fakesystemd systemd
+
 RUN (cd /lib/systemd/system/sysinit.target.wants/; for i in *; do [ $i == systemd-tmpfiles-setup.service ] || rm -f $i; done); \
 rm -f /lib/systemd/system/multi-user.target.wants/*; \
 rm -f /etc/systemd/system/*.wants/*; \
@@ -9,29 +8,23 @@ rm -f /lib/systemd/system/sockets.target.wants/*udev*; \
 rm -f /lib/systemd/system/sockets.target.wants/*initctl*; \
 rm -f /lib/systemd/system/basic.target.wants/*; \
 rm -f /lib/systemd/system/anaconda.target.wants/*;
-RUN yum -y install \
+
+RUN yum clean all && \
+    yum -y install epel-release && \
+    yum -y install \
     acl \
     asciidoc \
-    dbus-python \
     bzip2 \
-    epel-release \
+    dbus-python \
     file \
     git \
     iproute \
     make \
-    mercurial \
     mariadb-server \
+    mercurial \
     MySQL-python \
-    rpm-build \
-    rubygems \
-    subversion \
-    sudo \
-    unzip \
     openssh-clients \
     openssh-server \
-    which
-RUN yum -y install \
-    PyYAML \
     python-coverage \
     python-httplib2 \
     python-jinja2 \
@@ -41,7 +34,18 @@ RUN yum -y install \
     python-paramiko \
     python-pip \
     python-setuptools \
-    python-virtualenv
+    python-virtualenv \
+    PyYAML \
+    rpm-build \
+    rubygems \
+    subversion \
+    sudo \
+    unzip \
+    which \
+    && \
+    yum -y swap fakesystemd systemd && \
+    yum clean all
+
 RUN /usr/bin/sed -i -e 's/^\(Defaults\s*requiretty\)/#--- \1/'  /etc/sudoers
 RUN mkdir /etc/ansible/
 RUN /usr/bin/echo -e '[local]\nlocalhost ansible_connection=local' > /etc/ansible/hosts