ceph: make life easier on developers by handling in-tree runs

If <path-to-ceph> contains pybind and .libs:
- prepend <path-to-ceph>/pybind to PYTHONPATH
- append <path-to-ceph>/.libs to LD_LIBRARY_PATH if not already there
  and exec self so it takes effect

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>