Add v6 Option 59 to SUPPORTED_DHCP_OPTS_MAPPING

I have a usecase where I boot nova instances using an iPXE
image. For IPv6 setting option 59 did not work, but it seems
all that is needed to make the OVN DHCP service serve Option 59
is to add it to the supported opts mapping.

```
Configuring (net0 fa:16:3e:de:cf:88)................. ok
net0: 2620:cf:cf:cf02::10/64 gw fe80::f816:3eff:feca:5746
net0: fe80::f816:3eff:fede:cf88/64
Filename: http://[2620:cf:cf:cf02::fff0]:8081/boot-artifacts/agent.x86_64.ipxe
```

Closes-Bug: #2125715
Change-Id: I43bb10e45a589e947e27e0bc98af4cef5fc7a386
Signed-off-by: Harald Jensås <hjensas@redhat.com>

neutron/common/ovn/constants.py

@@ -189,14 +189,16 @@ SUPPORTED_DHCP_OPTS_MAPPING = {
         '210': 'path_prefix',
         '150': 'tftp_server_address',
         '255': 'next_server'},
-    6: {'server-id': 'server_id',
+    6: {'bootfile-name': 'bootfile_name',
+        'server-id': 'server_id',
         'dns-server': 'dns_server',
         'domain-search': 'domain_search',
         'ia-addr': 'ia_addr',
         '2': 'server_id',
         '5': 'ia_addr',
         '24': 'domain_search',
-        '23': 'dns_server'},
+        '23': 'dns_server',
+        '59': 'bootfile_name'},
 }
 
 # Baremetal specific DHCP options for VNIC_BAREMETAL ports

releasenotes/notes/ovn-support-dhcpv6-opt-59-a2cce695182285d8.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Use of DHCPv6 option 59 (bootfile-name) is now supported with OVN.
+    See bug: `2125715 <https://bugs.launchpad.net/neutron/+bug/2125715>`_.