Disable sg-core related tests by default

The sg-core is not required to run Telemetry services. Enabling tests
requiring sg-core by default is not backword-compatible and affects
testing in distributions(eg. RDO) or any users using tempest to
validate their deployments.

Change-Id: Ib08a42f22325d4029b5148f081e1b095fbfc723e
diff --git a/.zuul.yaml b/.zuul.yaml
index fdeb80f..9caf6d6 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -107,6 +107,8 @@
       devstack_local_conf:
         test-config:
           $TEMPEST_CONFIG:
+            service_available:
+              sg-core: True
             telemetry:
               disable_ssl_certificate_validation: True
       tempest_test_regex: telemetry_tempest_plugin
diff --git a/telemetry_tempest_plugin/config.py b/telemetry_tempest_plugin/config.py
index be3f6d4..7546834 100644
--- a/telemetry_tempest_plugin/config.py
+++ b/telemetry_tempest_plugin/config.py
@@ -32,7 +32,7 @@
                               help="Whether or not Gnocchi is expected to be"
                                    "available"),
                   cfg.BoolOpt('sg_core',
-                              default=True,
+                              default=False,
                               help="Whether or not sg-core is expected to be"
                                    "available")]