Fix rdo-release install

The URL for rdo-release package is version-less and redirects to latest
stable version. This becomes problematic when stacking older stable
versions as dependencies might not be met or newer and incompatibile
ones might get installed.

Closes-Bug: #1833696

Change-Id: Icb07dcb4c9a3950a3c31a3a8dcb8d0b4c713fdb1
diff --git a/stack.sh b/stack.sh
index 9bda79a..80a7d0e 100755
--- a/stack.sh
+++ b/stack.sh
@@ -331,7 +331,13 @@
     # Per the point above, it's a bunch of repos so starts getting a
     # little messy...
     if ! is_package_installed rdo-release ; then
-        yum_install https://rdoproject.org/repos/rdo-release.rpm
+        if [[ "$TARGET_BRANCH" == "master" ]]; then
+            yum_install https://rdoproject.org/repos/rdo-release.rpm
+        else
+            # Get latest rdo-release-$rdo_release RPM package version
+            rdo_release=$(echo $TARGET_BRANCH | sed "s|stable/||g")
+            yum_install https://rdoproject.org/repos/openstack-$rdo_release/rdo-release-$rdo_release.rpm
+        fi
     fi
 
     # Also enable optional for RHEL7 proper.  Note this is a silent