Fix sg-core test

Adding "http://" to the sg-core address fixes the sg-core test
on Ubuntu noble. I hope this is a small enough change to the default
to be safe to merge.

Change-Id: If3f67d3ec5c1b8c85362a7d79d857872bff042fe
diff --git a/telemetry_tempest_plugin/config.py b/telemetry_tempest_plugin/config.py
index e2e1766..bf546ee 100644
--- a/telemetry_tempest_plugin/config.py
+++ b/telemetry_tempest_plugin/config.py
@@ -81,7 +81,7 @@
                 help="Disable SSL certificate validation when running "
                      "scenario tests"),
     cfg.StrOpt('sg_core_service_url',
-               default="127.0.0.1:3000",
+               default="http://127.0.0.1:3000",
                help="URL to sg-core prometheus endpoint"),
     cfg.StrOpt('prometheus_service_url',
                default="127.0.0.1:9090",