Add optional repo enablement for RHEL 7

The RHEL 7 and RHEL 6 optional repos have different names. This enables
the correct repo depending on the release.

Change-Id: I6772c9903a5b9e98cb870d586629564126a6119a
diff --git a/stack.sh b/stack.sh
index e73b16a..d2b2330 100755
--- a/stack.sh
+++ b/stack.sh
@@ -246,7 +246,12 @@
 
     # ... and also optional to be enabled
     is_package_installed yum-utils || install_package yum-utils
-    sudo yum-config-manager --enable rhel-6-server-optional-rpms
+    if [[ $DISTRO =~ (rhel7) ]]; then
+        OPTIONAL_REPO=rhel-7-server-optional-rpms
+    else
+        OPTIONAL_REPO=rhel-6-server-optional-rpms
+    fi
+    sudo yum-config-manager --enable ${OPTIONAL_REPO}
 
 fi