Add tempest-list-plugins

Add the tempest-list-plugins cmd.

bp list-plugins

Change-Id: If28311bc2e8d29a97ee46d7d73edba2a93aed7ce
diff --git a/setup.cfg b/setup.cfg
index 4415063..b94a4f4 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -36,6 +36,7 @@
     init = tempest.cmd.init:TempestInit
     cleanup = tempest.cmd.cleanup:TempestCleanup
     run-stress = tempest.cmd.run_stress:TempestRunStress
+    list-plugins = tempest.cmd.list_plugins:TempestListPlugins
 oslo.config.opts =
     tempest.config = tempest.config:list_opts