Fix reference to nonexistent ssh_connect_method in config help

The compute.ssh_connect_method option was replaced by the
validation.connect_method option. There is another option's
help which references ssh_connect_method though so this
cleans that up to avoid further confusion.

Change-Id: Ifb71314badcb41190cdaa0c4e780c0c5583f0c7b
diff --git a/tempest/config.py b/tempest/config.py
index 6bae021..ea3ed5a 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -501,7 +501,7 @@
                 default=False,
                 help="Whether project networks can be reached directly from "
                      "the test client. This must be set to True when the "
-                     "'fixed' ssh_connect_method is selected."),
+                     "'fixed' connect_method is selected."),
     cfg.StrOpt('public_network_id',
                default="",
                help="Id of the public network that provides external "