Fix wrong epel-release-7* package URL

When "epel" element is used during a build process
with "rhel7" distribution, the build failed
because the "epel-release-7*" package cannot be
installed.

The reason is because the URL is not correct, it
should be:
  URL=$BASE_URL/$RELEASE/x86_64/Packages/e/

Change-Id: I90c26892361f7611645b85f2eddc949b2f0d76fc
Closes-Bug: #1735547

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

@@ -21,7 +21,7 @@ else
     case "$DISTRO_NAME" in
         rhel7)
             RELEASE=7
-            URL=$BASE_URL/$RELEASE/x86_64/e/
+            URL=$BASE_URL/$RELEASE/x86_64/Packages/e/
             ;;
         *)
             echo "$DISTRO_NAME is not supported"