Test for id_username instead of "Log In"

The string "Log In" is maybe no available when customizing the login
page. It's much more unlikely that the form field id_username changes in
an customized login page.

Change-Id: Ifadace32b4e8433d9efb591ec045d0854aa557be
diff --git a/tempest/scenario/test_dashboard_basic_ops.py b/tempest/scenario/test_dashboard_basic_ops.py
index 35f6689..1313050 100644
--- a/tempest/scenario/test_dashboard_basic_ops.py
+++ b/tempest/scenario/test_dashboard_basic_ops.py
@@ -65,7 +65,7 @@
 
     def check_login_page(self):
         response = urllib2.urlopen(CONF.dashboard.dashboard_url)
-        self.assertIn("Log In", response.read())
+        self.assertIn("id_username", response.read())
 
     def user_login(self, username, password):
         self.opener = urllib2.build_opener(urllib2.HTTPCookieProcessor())