Make rdo-release install safe

Like the epel element, this may be applied to a system that already
has rdo-release installed.  If that happens yum exits non-zero
because for some reason it considers "nothing to do" to be an error.

This simply checks for the existence of the rdo-release rpm before
attempting to install it.

Change-Id: I5867ca074cc1afa843ef1e0f85c1c9b6f2deb403