Add option to set EPEL mirror

In some environments it may be desirable to set the EPEL
mirror to use. Add this ability.

Change-Id: Iefa22bde7a64b40e6d36ba39efa9102de9393e06

elements/epel/README.rst

@@ -3,3 +3,11 @@ epel
 ====
 This element installs the Extra Packages for Enterprise Linux (EPEL)
 repository GPG key as well as configuration for yum.
+
+DIB_EPEL_MIRROR:
+   :Required: No
+   :Default: None
+   :Description: To use a EPEL Yum mirror, set this variable to the mirror URL
+                 before running bin/disk-image-create. This URL should point to
+                 the directory containing the ``5/6/7`` directories.
+   :Example: ``DIB\_EPEL\_MIRROR=http://dl.fedoraproject.org/pub/epel``

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

@@ -29,3 +29,10 @@ 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
+
+
+DIB_EPEL_MIRROR=${DIB_EPEL_MIRROR:-}
+[ -n "$DIB_EPEL_MIRROR" ] || exit 0
+
+# Set the EPEL mirror to use
+sed -e "s|^#baseurl=http://download.fedoraproject.org/pub/epel|baseurl=$DIB_EPEL_MIRROR|;/^mirrorlist=/d" -i /etc/yum.repos.d/epel.repo