Fix deprecated_group of rpc_conn_pool_size

When rpc_conn_pool_size have been moved from amqp.py to base.py:

  87137e7af0

We loose the deprecated_group, this change reintroduces it.

Change-Id: I8cdea7f042afebcc162bafef881ebe61a1cac989

oslo_messaging/_drivers/base.py

@@ -23,6 +23,7 @@ from oslo_messaging import exceptions
 base_opts = [
     cfg.IntOpt('rpc_conn_pool_size',
                default=30,
+               deprecated_group='DEFAULT',
                help='Size of RPC connection pool.'),
 ]