Update cfg option network_for_ssh's help msg

   cfg option network_for_ssh's help msg says "use_floatingip_for_ssh
=true", but now use_floatingip_for_ssh is deprecated, and its value
cannot be "true" or "false" any longer, so it should refer to
connect_method instead.

Change-Id: I4690c8dc1ad0c14ea3e62e54314044e13c688581
Closes-Bug: #1581376
diff --git a/tempest/config.py b/tempest/config.py
index 3e0f28f..997ee2c 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -672,7 +672,7 @@
     cfg.StrOpt('network_for_ssh',
                default='public',
                help="Network used for SSH connections. Ignored if "
-                    "use_floatingip_for_ssh=true or run_validation=false.",
+                    "connect_method=floating or run_validation=false.",
                deprecated_opts=[cfg.DeprecatedOpt('network_for_ssh',
                                                   group='compute')]),
 ]