Fix up EPEL element

For whatever reason, RHEL identifies itself with DISTRO "rhel" for 6
and "rhel7" for 7, but centos just uses "centos" and DIB_RELEASE.  So
this was wrong and installing EPEL6 on centos7.

But we can simplify it completely for centos because that comes with
the epel-release package already included.

Change-Id: I2b8f5d30b850fef46b4a5ba32a917abcbf25932c

elements/epel/pre-install.d/05-rpm-epel-release

@@ -6,19 +6,24 @@ fi
 set -eu
 set -o pipefail
 
+if [[ ${DISTRO_NAME} =~ "centos" ]]; then
+    # Centos has "epel-release" in extras, which is default enabled.
+    yum install -y epel-release
+else
+    # For RHEL, we have to scrape the download page to find the latest
+    # release and install that
     [ -n "$ARCH" ]
-
     if [ 'amd64' = "$ARCH" ] ; then
         ARCH="x86_64"
     fi
 
     BASE_URL=${DIB_EPEL_MIRROR:-https://dl.fedoraproject.org/pub/epel}
     case "$DISTRO_NAME" in
-    rhel7|centos7)
+        rhel7)
             RELEASE=7
             URL=$BASE_URL/$RELEASE/x86_64/e/
             ;;
-    rhel|centos)
+        rhel)
             RELEASE=6
             URL=$BASE_URL/$RELEASE/$ARCH/
             ;;
@@ -29,7 +34,7 @@ case "$DISTRO_NAME" in
     esac
     PKG_NAME=$(wget -q $URL -O - |grep -oE "(href=\"epel-release-$RELEASE-[0-9,.].*)" | cut -d'"' -f2)
     rpm -q epel-release || yum install -y $URL/$PKG_NAME
-
+fi
 
 DIB_EPEL_MIRROR=${DIB_EPEL_MIRROR:-}
 [ -n "$DIB_EPEL_MIRROR" ] || exit 0