Marking the login_url option as "deprecated_for_removal"

https://review.openstack.org/#/c/212239 has been successfully
merged and now we don't need the login_url option because only
the dashboard_url option is used in the dashboard test. This patch
is intended to mark CONF.dashboard.login_url as deprecated_for_removal.

Change-Id: I4195478d93da605218fddf9aa896e2804ab442ac
diff --git a/tempest/config.py b/tempest/config.py
index 0262d1b..e495695 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -881,7 +881,8 @@
                help="Where the dashboard can be found"),
     cfg.StrOpt('login_url',
                default='http://localhost/auth/login/',
-               help="Login page for the dashboard"),
+               help="Login page for the dashboard",
+               deprecated_for_removal=True),
 ]