remove python-backports on rhos install

python-backports is installed as part of cloud-init.
python-backports.x86_64 0:1.0-3.el6 is known to cause issues w/ glance.
Once cloud init is executed on the instance it can be removed.

Change-Id: Ie1c21c54f3c06b352d8c7f418c07bc8dae7a4b9e