Fix Usage of rdo-release rpm

rdo-release.el8.rpm rpm points to latest RDO release,
so use it for master, for stable releases use corresponding
release rpm.

Change-Id: I508eceb00d7501ffcfac73d7bc2272badb241494
diff --git a/stack.sh b/stack.sh
index c439a72..e3d67f5 100755
--- a/stack.sh
+++ b/stack.sh
@@ -300,10 +300,14 @@
 }
 
 function _install_rdo {
-    # NOTE(ianw) 2020-04-30 : when we have future branches, we
-    # probably want to install the relevant branch RDO release as
-    # well.  But for now it's all master.
-    sudo dnf -y install https://rdoproject.org/repos/rdo-release.el8.rpm
+    if [[ "$TARGET_BRANCH" == "master" ]]; then
+        # rdo-release.el8.rpm points to latest RDO release, use that for master
+        sudo dnf -y install https://rdoproject.org/repos/rdo-release.el8.rpm
+    else
+        # For stable branches use corresponding release rpm
+        rdo_release=$(echo $TARGET_BRANCH | sed "s|stable/||g")
+        sudo dnf -y install https://rdoproject.org/repos/openstack-${rdo_release}/rdo-release-${rdo_release}.el8.rpm
+    fi
     sudo dnf -y update
 }