Add a [placement] section into Neutron config

It appears to be that the Neutron Segment plugin needs to instantiate a
placement service client which requires credentials and other connection
details:
https://opendev.org/openstack/neutron/src/branch/stable/ussuri/neutron/services/segments/plugin.py#L188

Otherwise, the following exception can be seen periodically in the
Neutron log:

keystoneauth1.exceptions.auth_plugins.MissingAuthPlugin:
 An auth plugin is required to determine endpoint URL

Change-Id: I480292a1c74c376db5c9797b1fcc8469e0e5507a

snap-overlay/snap-openstack.yaml

@@ -70,6 +70,7 @@ setup:
     placement.conf.d.database.conf.j2: "{snap_common}/etc/placement/placement.conf.d/database.conf"
     neutron.keystone.conf.j2: "{snap_common}/etc/neutron/neutron.conf.d/keystone.conf"
     neutron.nova.conf.j2: "{snap_common}/etc/neutron/neutron.conf.d/nova.conf"
+    neutron.placement.conf.j2: "{snap_common}/etc/neutron/neutron.conf.d/placement.conf"
     neutron.database.conf.j2: "{snap_common}/etc/neutron/neutron.conf.d/database.conf"
     neutron.conf.d.rabbitmq.conf.j2: "{snap_common}/etc/neutron/neutron.conf.d/rabbitmq.conf"
     neutron_ovn_metadata_agent.ini.j2: "{snap_common}/etc/neutron/neutron_ovn_metadata_agent.ini"

snap-overlay/templates/neutron.placement.conf.j2

@@ -0,0 +1,9 @@
+[placement]
+auth_url = http://{{ control_ip }}:5000
+auth_type = password
+project_domain_name = default
+user_domain_name = default
+region_name = {{ region_name }}
+project_name = service
+username = placement
+password = {{ placement_password }}