Remove unused config option image_alt_ssh_user

It's not used in the code and the corresponding
image_alt_ssh_password is no longer there so remove the option.

Change-Id: I9d3b3cd6e481edf4c1092335736acc04f1127adb
diff --git a/tempest/config.py b/tempest/config.py
index 0cda018..61cdaa6 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -237,10 +237,6 @@
     cfg.StrOpt('image_ssh_password',
                default="password",
                help="Password used to authenticate to an instance."),
-    cfg.StrOpt('image_alt_ssh_user',
-               default="root",
-               help="User name used to authenticate to an instance using "
-                    "the alternate image."),
     cfg.IntOpt('build_interval',
                default=1,
                help="Time in seconds between build status checks."),