As far as I remember, the br-ex situation was tricky on rdo. You could read something about it here:
https://openstack.redhat.com/Neutron_with_existing_external_network (could be obsolete)
At some point, rdo was able to add br-ex, but did it wrong, leaving users without connectivity, there was some work/patch on the way to fix how puppet handled br-ex setup (as per document above) but I don't know what happened to it.
In my own experience (haven't tested packstack since icehouse), having br-ex already configured with an slave connected to the external network prior to running packstack was the best way to have everything working as it was supposed to after rdo finished