Add enable flag for RDO yum repository on RHEL

Each user can have their own specific repository for OpenStack.
Under this scenario, a flag is required to let user disable
the RDO yum repository.

Co-Authored-By: Matt Odden <mrodden@us.ibm.com>
Change-Id: Ifb271b5e11f3e26c5a217fcdd64ad546e378447f
Closes-Bug: #1269348
Merge "Add enable flag for RDO yum repository on RHEL"