Revert "centos7: don't install EPEL"

This reverts commit 37c26280bf.

This was originally removed so that EPEL would not be installed in
our nodepool images.  We no longer run this script when creating
those images, so it can be safely restored.  It should be restored
because without this, we are unable to spin up a cgit server
(which requires epel).

Change-Id: I93e6487ab9a2eae5f927abf43aea7e3e19c5ac9e

install_puppet.sh

@@ -135,6 +135,20 @@ function setup_puppet_fedora {
 }
 
 function setup_puppet_rhel7 {
+    # install a bootstrap epel repo to install latest epel-release
+    # package (which provides correct gpg keys, etc); then remove
+    # boostrap
+    cat > /etc/yum.repos.d/epel-bootstrap.repo <<EOF
+[epel-bootstrap]
+name=Bootstrap EPEL
+mirrorlist=https://mirrors.fedoraproject.org/mirrorlist?repo=epel-7&arch=\$basearch
+failovermethod=priority
+enabled=0
+gpgcheck=0
+EOF
+    yum --enablerepo=epel-bootstrap -y install epel-release
+    rm -f /etc/yum.repos.d/epel-bootstrap.repo
+
     _systemd_update
     yum update -y