Add dhcpagentscheduler API extension to the ML2/OVN extensions

In most typical use cases ML2/OVN backend don't needs to run DHCP agent
as OVN provides DHCP functionality natively. But there are some use
cases like Baremetal provisioning over IPv6 or Spine Leaf architecture
with DHCP relays where DHCP agent is necessary and it can work perfectly
fine with ML2/OVN backend.
The problem was that dhcpagentscheduler API extension wasn't listed as
supported by the OVN backend so it was filtered out from the list of
supported extensions during start of the neutron server. This caused
problems with API to get/set/delete networks to/from DHCP agent.

This patch adds this API extension to the list of the extensions
supported by the OVN driver to fix that issue.

Depends-On: https://review.opendev.org/c/openstack/tempest/+/898090

Closes-bug: #2038655

Change-Id: I09a37ca451d44607b7dde344c93ace060c7bda01
(cherry picked from commit f006d29)

Author: slawqo

neutron/common/ovn/extensions.py

@@ -21,6 +21,7 @@
 from neutron_lib.api.definitions import bgp_4byte_asn
 from neutron_lib.api.definitions import bgp_dragentscheduler
 from neutron_lib.api.definitions import default_subnetpools
+from neutron_lib.api.definitions import dhcpagentscheduler
 from neutron_lib.api.definitions import dns
 from neutron_lib.api.definitions import dns_domain_keywords
 from neutron_lib.api.definitions import dns_domain_ports
@@ -117,6 +118,7 @@
     portbindings.ALIAS,
     pbe_ext.ALIAS,
     default_subnetpools.ALIAS,
+    dhcpagentscheduler.ALIAS,
     dns.ALIAS,
     external_net.ALIAS,
     extra_dhcp_opt.ALIAS,