Use 'os_primary' property insead of deprecated 'os'

Read-only property 'os' has moved to 'os_primary' in version 'Pike' and
will be removed in version 'Queens'

Change-Id: Ice03458c69d0e107dac6f6735391c3e8567cf855
diff --git a/tempest_horizon/tests/scenario/test_dashboard_basic_ops.py b/tempest_horizon/tests/scenario/test_dashboard_basic_ops.py
index e9f72d6..a7cd4dd 100644
--- a/tempest_horizon/tests/scenario/test_dashboard_basic_ops.py
+++ b/tempest_horizon/tests/scenario/test_dashboard_basic_ops.py
@@ -137,7 +137,7 @@
 
     @decorators.idempotent_id('4f8851b1-0e69-482b-b63b-84c6e76f6c80')
     def test_basic_scenario(self):
-        creds = self.os.credentials
+        creds = self.os_primary.credentials
         self.check_login_page()
         self.user_login(creds.username, creds.password)
         self.check_home_page()