Fix incorrect config of linuxbridge multiple external networks

multiple external networks are supported by linuxbridge and OVS.
Currently the config template only works for OVS

Closes-Bug: #1863935
Change-Id: I9da331e007c25c4a760839c566831769a68507a9

ansible/roles/neutron/templates/linuxbridge_agent.ini.j2

@@ -4,7 +4,9 @@ extensions = {{ neutron_agent_extensions|map(attribute='name')|join(',') }}
 {% endif %}
 
 [linux_bridge]
-physical_interface_mappings = physnet1:{{ neutron_external_interface }}
+{% if inventory_hostname in groups["network"] or (inventory_hostname in groups["compute"] and computes_need_external_bridge | bool ) %}
+physical_interface_mappings = {% for interface in neutron_external_interface.split(',') %}physnet{{ loop.index0 + 1 }}:{{ interface }}{% if not loop.last %},{% endif %}{% endfor %}
+{% endif %}
 
 [securitygroup]
 firewall_driver = neutron.agent.linux.iptables_firewall.IptablesFirewallDriver

ansible/roles/neutron/templates/ml2_conf.ini.j2

@@ -24,7 +24,7 @@ network_vlan_ranges =
 {% if enable_ironic | bool %}
 flat_networks = *
 {% else %}
-flat_networks = {% for bridge in neutron_bridge_name.split(',') %}physnet{{ loop.index0 + 1 }}{% if not loop.last %},{% endif %}{% endfor %}
+flat_networks = {% for interface in neutron_external_interface.split(',') %}physnet{{ loop.index0 + 1 }}{% if not loop.last %},{% endif %}{% endfor %}
 {% endif %}
 
 [ml2_type_vxlan]

releasenotes/notes/linux-bridge-multi-fe8576616fb7d373.yaml

+---
+fixes:
+  - |
+    Fixes an issue with Neutron ``linuxbridge`` ML2 agent when
+    ``neutron_external_interface`` includes multiple interfaces. `LP#1863935
+    <https://launchpad.net/bugs/1863935>`__