Rename core fixtures to match the drivetrain component names

The following changes:
  common_services_deployed -> core_deployed
  sl_deployed -> stacklight_deployed

Change-Id: I6e505d2ef96053b19b2d43b0214f5a30f71ae8ea
diff --git a/tcp_tests/fixtures/oss_fixtures.py b/tcp_tests/fixtures/oss_fixtures.py
index 95bbc54..6162e27 100644
--- a/tcp_tests/fixtures/oss_fixtures.py
+++ b/tcp_tests/fixtures/oss_fixtures.py
@@ -88,8 +88,8 @@
                        oss_deployed):
     """Fixture to get or install SL and OpenStack services on environment
 
-    Uses fixtures openstack_deployed and sl_deployed, with 'sl_deployed'
-    top-level snapshot.
+    Uses fixtures openstack_deployed and stacklight_deployed,
+    with 'stacklight_deployed' top-level snapshot.
 
     Returns SLManager instance object
     """