Filter subnets by "enable_dhcp" flag using the correct type

While other SQL engines can compare interger and boolean types,
PostgreSQL needs explicit casting to compare variables. Method
"_sync_subnet_dhcp_options" is currently raising the following
error:
  operator does not exist: boolean = integer

Closes-Bug: #2004581
Change-Id: I715029c311c4516f3212054c5c72533b12fd0986
(cherry picked from commit 61b2917)

Author: ralonsoh

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_db_sync.py

@@ -769,7 +769,7 @@ def _sync_subnet_dhcp_options(self, ctx, db_networks,
         LOG.debug('OVN-NB Sync DHCP options for Neutron subnets started')
 
         db_subnets = {}
-        filters = {'enable_dhcp': [1]}
+        filters = {'enable_dhcp': [True]}
         for subnet in self.core_plugin.get_subnets(ctx, filters=filters):
             if (subnet['ip_version'] == constants.IP_VERSION_6 and
                     subnet.get('ipv6_address_mode') == constants.IPV6_SLAAC):