ovn: Add dns_servers configuration support

networking-ovn optionally expects list of dns server to be defined in the
config -'ovn/dns_servers'. These will be included in the 'dns_server' DHCP
option field when the VMs send DHCP request if the subnet's dns_nameservers field
is not defined by the user.

This patch sets the hieradata - 'neutron::plugins::ml2::ovn::dns_servers' to the
defined OVNDnsServers t-h-t param.

Change-Id: I80574f7badfcc618254266051c8d6661c08e2be0
Closes-bug: #1774052
(cherry picked from 371e5d6264)
Conflicts:
       environments/services/neutron-ovn-hw-offload.yaml

environments/services-docker/neutron-ovn-dvr-ha.yaml

@@ -27,3 +27,4 @@ parameter_defaults:
   NeutronEnableDVR: true
   ControllerParameters:
     OVNCMSOptions: "enable-chassis-as-gw"
+  OVNDnsServers: []

environments/services-docker/neutron-ovn-dvr.yaml

@@ -27,3 +27,4 @@ parameter_defaults:
   NeutronEnableDVR: true
   ControllerParameters:
     OVNCMSOptions: "enable-chassis-as-gw"
+  OVNDnsServers: []

environments/services-docker/neutron-ovn-ha.yaml

@@ -31,3 +31,4 @@ parameter_defaults:
     OVNCMSOptions: "enable-chassis-as-gw"
   NetworkerParameters:
     OVNCMSOptions: "enable-chassis-as-gw"
+  OVNDnsServers: []

environments/services-docker/neutron-ovn.yaml

@@ -31,3 +31,4 @@ parameter_defaults:
     OVNCMSOptions: "enable-chassis-as-gw"
   NetworkerParameters:
     OVNCMSOptions: "enable-chassis-as-gw"
+  OVNDnsServers: []

puppet/services/neutron-plugin-ml2-ovn.yaml

@@ -82,6 +82,10 @@ parameters:
     constraints:
       - allowed_values:
           - geneve
+  OVNDnsServers:
+    default: []
+    description: List of servers to use as as dns forwarders
+    type: comma_delimited_list
 
 resources:
 
@@ -112,6 +116,7 @@ outputs:
             neutron::server::qos_notification_drivers: {get_param: OVNQosDriver}
             neutron::plugins::ml2::max_header_size: {get_param: NeutronGeneveMaxHeaderSize}
             neutron::plugins::ml2::ovn::dvr_enabled: {get_param: NeutronEnableDVR}
+            neutron::plugins::ml2::ovn::dns_servers: {get_param: OVNDnsServers}
             neutron::plugins::ml2::tenant_network_types: {get_param: NeutronNetworkType}
       step_config: |
         include ::tripleo::profile::base::neutron::plugins::ml2

releasenotes/notes/ovn-set-dns-servers-config-option-e8f91ad2a05df9a9.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - Provides the option to define a set of DNS servers which will be
+    configured in the 'ovn' section of etc/neutron/plugins/ml2_conf.ini.
+    These DNS servers will be used as DNS forwarders for the VMs if a neutron
+    subnet is not defined with 'dns_nameservers' option.