Rdo-release install failure under proxy

In RHEL under proxy, installation by stack.sh failed
because of rdo-release.rpm install failure.

This patch fixes install command for rdo-release.rpm
because it is caused by lack of http(s)_proxy.

Change-Id: I176d1e140f52e1bb0343170ba4d90c06b98d5a99
Closes-Bug: #1536478
diff --git a/stack.sh b/stack.sh
index e9831d5..6f43785 100755
--- a/stack.sh
+++ b/stack.sh
@@ -290,7 +290,7 @@
     sudo yum-config-manager --enable rhel-7-server-optional-rpms
 
     # install the lastest RDO
-    sudo yum install -y https://rdoproject.org/repos/rdo-release.rpm
+    is_package_installed rdo-release || yum_install https://rdoproject.org/repos/rdo-release.rpm
 
     if is_oraclelinux; then
         sudo yum-config-manager --enable ol7_optional_latest ol7_addons ol7_MySQL56