container ovs-agent, ensure br-ex exists

container ovs-agent, ensure br-ex exists

Currently the container neutron-ovs-agent is stuck in a restart loop
in many environments because the bridge br-ex is missing.

This bridge is created by running the puppet class
neutron::agents::ml2::ovs but limiting that run to tag
neutron::plugins::ovs::bridge.

The hiera neutron::agents::ml2::ovs::bridge_mappings should already
exists to create the bridge with the required settings.

This change should ensure br-ex exists after step 3.

Since br-ex is created regardless of the chosen network config,
environments/docker-network.yaml is not longer required. It can be
deleted once there are no more references to it in CI and
documentation.

Change-Id: Ie425148b0ad0f38e149c5fa0a97d98ec35d0a5bb
Closes-Bug: #1699261
Closes-Bug: #1691403
Closes-Bug: #1689556
(cherry picked from commit 76f130d6e8f7434433b2602af9794f1e9c742e1f)