[OVN] Use elevated context to retrieve subnet in router port configuration

The method ``_get_nets_and_ipv6_ra_confs_for_router_port`` can be called
from a non-admin user request, when updating or creating a local router
port. If the router external gateway network is "external" (as it should
be) but is not explicitly shared (a network RBAC with action
"access_as_shared"), the user won't retrieve the corresponding subnet.

NOTE: is is *not* needed to apply both "access_as_shared" and
"access_as_external" RBACs to a network. Please read c#1 in the LP bug
for more context.

Related-Bug: #2051831
Change-Id: I161f1a6021c0da2d0063f8cb249b3bb9d7b6d5ae
(cherry picked from commit 70e51eb)

Author: ralonsoh

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

@@ -1203,7 +1203,13 @@ def _get_nets_and_ipv6_ra_confs_for_router_port(self, context, port):
 
         for fixed_ip in port_fixed_ips:
             subnet_id = fixed_ip['subnet_id']
-            subnet = self._plugin.get_subnet(context, subnet_id)
+            # NOTE(ralonsoh): it is needed to use the "admin" context here to
+            # retrieve the subnet. The subnet object is not handling correctly
+            # the RBAC filtering because is not filtering by
+            # "access_as_external", as network object is doing in
+            # ``_network_filter_hook``. See LP#2051831.
+            # TODO(ralonsoh): once LP#2051831 is fixed, remove "elevated()".
+            subnet = self._plugin.get_subnet(context.elevated(), subnet_id)
             cidr = netaddr.IPNetwork(subnet['cidr'])
             networks.add("%s/%s" % (fixed_ip['ip_address'],
                                     str(cidr.prefixlen)))