Add config option for ovn_emit_need_to_frag

Add a charm config option to enable/disable ovn_emit_need_to_frag.
By default, the value is set to "True" as ovn_emit_need_to_frag
is enabled by default.

Closes-Bug: #1947391
Change-Id: I58fe365ce862430e172ebedf80ebaef9807c910c

src/config.yaml

@@ -121,3 +121,15 @@ options:
         - ntp_server:,wpad:1.2.3.5 - Unset ntp_server and set wpad
 
         See the ovn-nb(5) man page for available options.
+  enable-emit-need-to-frag:
+    type: boolean
+    default: True
+    description: >
+      When enabled, the length of packets traversing router gateway ports 
+      will be checked, and if necessary ICMPv4 type 3 (Destination 
+      Unreachable) code 4 (Fragmentation Needed and Don't Fragment was Set)
+      or ICMPv6 type 2 (Packet Too Big) code 0 (no route to destination)
+      packets will be generated.
+
+      Note that a performance penalty may incur on older kernel versions (<= 5.2)
+      or if hardware acceleration does not support the ``check_pkt_len`` action.

src/reactive/neutron_api_plugin_ovn_handlers.py

@@ -133,7 +133,7 @@ def configure_neutron():
                 # version of 5.2 or greater.  Enabled by default
                 # because charm supports focal or newer which has a
                 # minimum kernel version of 5.4.
-                ('ovn_emit_need_to_frag', True),
+                ('ovn_emit_need_to_frag', options.enable_emit_need_to_frag),
                 # NOTE(fnordahl): will be used on chassis with DPDK enabled
                 #
                 # Neutron will make per chassis decisions based on chassis

unit_tests/test_reactive_neutron_api_plugin_ovn_handlers.py

@@ -145,6 +145,7 @@ class TestOvnHandlers(test_utils.PatchHelper):
         options.ovn_l3_scheduler = self.pmock('aSched')
         options.ovn_metadata_enabled = self.pmock('aMetaData')
         options.enable_distributed_floating_ip = self.pmock('dont')
+        options.enable_emit_need_to_frag = self.pmock(False)
         options.dns_servers = self.pmock('dns1 dns2')
         options.geneve_vni_ranges = self.pmock('vnia:vniA vnib:vniB')
         options.dhcp_default_lease_time = self.pmock(42)
@@ -186,7 +187,7 @@ class TestOvnHandlers(test_utils.PatchHelper):
                                 ('dhcp_default_lease_time', 42),
                                 ('ovn_dhcp4_global_options', 'a:A4,b:B4'),
                                 ('ovn_dhcp6_global_options', 'a:A6,b:B6'),
-                                ('ovn_emit_need_to_frag', True),
+                                ('ovn_emit_need_to_frag', False),
                                 ('vhost_sock_dir', '/run/libvirt-vhost-user')
                             ],
                             'ml2_type_geneve': [