Make run_stress.py script an entry point

With change f8b816af0751b10cc20b0ab456ca334150fcdf5f we started
packaging binaries using pbr entry points. The run_stress.py script
previously lived in tempest.stress, howver since it is intended to be
run as a binary the proper place for it is now tempest.cmd and using
an entry-point it will be easier to run it outside of the tempest
tree.

Change-Id: I96b2c7915875f4f9d7869b8728bf2c6f6ccbeec9
diff --git a/setup.cfg b/setup.cfg
index 339da12..5c62710 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -21,6 +21,7 @@
 console_scripts =
     verify-tempest-config = tempest.cmd.verify_tempest_config:main
     javelin2 = tempest.cmd.javelin:main
+    run-tempest-stress = tempest.cmd.run_stress:main
 
 [build_sphinx]
 all_files = 1