cl2/density: add vanilla workload to scheduler suite

The vanilla workload is the same workload as the default config.yaml
file. However, it is included again here for two reasons:

1. Everything else is kept constant if we use the scheduler suite
   instead of a separate job for the vanilla workload (even though the
   two jobs are very similar right now).

2. Convenience: it's easier to switch between different workloads whilst
   maintaining the same metrics in perf-dash if the job name doesn't
   have to change.

Signed-off-by: Adhityaa Chandrasekar <adtac@google.com>

clusterloader2/testing/density/scheduler-suite.yaml

@@ -1,3 +1,7 @@
+- identifier: vanilla
+  configPath: testing/density/config.yaml
+  overridePaths: []
+
 - identifier: pod-affinity
   configPath: testing/density/config.yaml
   overridePaths: