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