Packstack: packstack is not installing the latest python-backports

This is caused by the repo priority in the rdo-release repo
If epel is disabled, it will install

Change-Id: I489c4d52c451ffe6bdd0d77d5c217ec23095bcb7
packstack: python-backports needs to be updated after rdo-release is installed

Change-Id: I1928a240c71045e160aa165848c0f50aace1f1e5