Use six.seraise() instead of `raise exc, val, tb`

Python 3 doesent support odl `raise exception, value, traceback` syntax,
so we should use six.reraise() to make code compatible with python 3.

Related to blueprint make-python3-compatible

Change-Id: If66983d94667a3d3b0e809d4c0767b0829394f67
Merge "Use six.seraise() instead of `raise exc, val, tb`"