Move st2client setup from pbr to setuptools

Use default setuptools instead of pbr for more control on versioning. Remove pbr from requirements.
This will only work correctly if `setup.py` is ran from the root directory.

This is probably not a big deal since this is usually the case, but `os.path.dirname(os.path.realpath(__file__))`, might be a bit more robust if `setup.py` ever needs to be called outside the root directory.