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/requirements.txt b/requirements.txt
index 60dee97..be825f0 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,6 +2,7 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 pbr<2.0,>=0.11
+cliff>=1.13.0 # Apache-2.0
 anyjson>=0.3.3
 httplib2>=0.7.5
 jsonschema!=2.5.0,<3.0.0,>=2.0.0