Use functools.wrap() instead of custom implementation

In openstack.db.sqlalchemy.session.py:_wrap_db_err() we used
custom implementation of functools.wrap(). So this patch remove
custom implementation and use functools.wrap().

Closes-Bug: #1119726

Change-Id: Ic92658647bc37d81a819581128fec675ad545c4a
Merge "Use functools.wrap() instead of custom implementation"