Add cliff based common cli entrypoint

This commit adds the basic framework to tempest for a unified modular
CLI endpoint using cliff. As of right now this entry-point doesn't
really do anything but exist. It will be expanded in the future first
all the existing commands will be refactored to be used through this.
Secondly a new interface and workflow for running tempest will be
created on top of this.

Co-Authored-By: Matthew Treinish <mtreinish@kortar.org>

Change-Id: I1ff8d22c120dbc81d812f1f107db7c2d9b15a505
Implements: blueprint tempest-cli-improvements
diff --git a/setup.cfg b/setup.cfg
index 3a14bba..5c78632 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -25,6 +25,7 @@
     run-tempest-stress = tempest.cmd.run_stress:main
     tempest-cleanup = tempest.cmd.cleanup:main
     tempest-account-generator = tempest.cmd.account_generator:main
+    tempest = tempest.cmd.main:main
 
 oslo.config.opts =
     tempest.config = tempest.config:list_opts