Make image_ssh_password a secret so it does not show in console

We are doing this for downstream testing that use custom images
and in case you are not a cubs fan :)

Change-Id: I5860dc67233ce3135daf11b7901b93547ba1f30a
diff --git a/tempest/config.py b/tempest/config.py
index 1699c7d..204d977 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -833,7 +833,8 @@
                help="User name used to authenticate to an instance."),
     cfg.StrOpt('image_ssh_password',
                default="password",
-               help="Password used to authenticate to an instance."),
+               help="Password used to authenticate to an instance.",
+               secret=True),
     cfg.StrOpt('ssh_shell_prologue',
                default="set -eu -o pipefail; PATH=$$PATH:/sbin:/usr/sbin;",
                help="Shell fragments to use before executing a command "