Description of problem:

Deploying bridge mappings in the overcloud with rdo-manager does not work since the Neutron  openvswitch plugin's file (/etc/neutron/plugins/ml2/openvswitch_agent.ini) does not gets populated with the right option.
The option that needs modification, supposing you want to associate datacentre to a dedicated bridge for the floating ip, is this one:

bridge_mappings = datacentre:br-floating

This is not happening in Liberty (the file gets populated in the right way) and in Kilo the location of the file and its name are different, in fact the file is /etc/neutron/plugins/openvswitch/ovs_neutron_plugin.ini.

Version-Release number of selected component (if applicable):

CentOS Linux release 7.2.1511 (Core)
instack-undercloud-3.0.1-dev54.el7.centos.noarch
openstack-tripleo-heat-templates-1.0.1-dev45.el7.centos.noarch

How reproducible:

Deploy an overcloud with a command similar to this one:

openstack overcloud deploy --templates [...] -e /usr/share/openstack-tripleo-heat-templates/environments/puppet-pacemaker.yaml -e /usr/share/openstack-tripleo-heat-templates/environments/network-isolation.yaml -e /usr/share/openstack-tripleo-heat-templates/environments/net-single-nic-with-vlans.yaml -e /home/stack/network-environment.yaml --neutron-bridge-mappings datacentre:br-floating

Use a /home/stack/network-environment.yaml with contents similar to this:

...
...
parameter_defaults:
...
...
  NeutronExternalNetworkBridge: "''"

parameters:
  NeutronBridgeMappings: "datacentre:br-floating"

Actual results:

File /etc/neutron/plugins/ml2/openvswitch_agent.ini is not populated with the mappings settings.

Expected results:

File gets populated.

Additional info:

As a workaround is possible to manually change the settings, restart the neutron service via pcs and then start creating networks/router/floatingip. Note that if you already created some of these elements you need to delete and create them as new in order to have them running.