xdsinterop: extend the ports to use (#25881)

This is to add more ports for forwarding-rule.

It's in theory not necessary, because forwarding-rule doesn't need to use the
same port as the services. This is a limitation of the test framework, and can
be fixed in the future.

tools/run_tests/run_xds_tests.py

@@ -199,7 +199,7 @@ argp.add_argument('--network',
                   default='global/networks/default',
                   help='GCP network to use')
 argp.add_argument('--service_port_range',
-                  default='8080:8110',
+                  default='8080:8280',
                   type=parse_port_range,
                   help='Listening port for created gRPC backends. Specified as '
                   'either a single int or as a range in the format min:max, in '