Modify the help message for getting identity extensions

'keystone discover' is no longer valid to get identity
extensions, this is to change to use 'openstack extension
list --identity' in help message in config.py

Change-Id: Icfdf0fc9df67f697a52b82b55c4e4a3f63f447d7
diff --git a/tempest/config.py b/tempest/config.py
index 0743220..b14d4fd 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -227,7 +227,8 @@
                 help="A list of enabled identity extensions with a special "
                      "entry all which indicates every extension is enabled. "
                      "Empty list indicates all extensions are disabled. "
-                     "To get the list of extensions run: 'keystone discover'"),
+                     "To get the list of extensions run: "
+                     "'openstack extension list --identity'"),
     # TODO(rodrigods): This is a feature flag for bug 1590578 which is fixed
     # in Newton and Ocata. This option can be removed after Mitaka is end of
     # life.