Add missing DBDuplicateEntry

The session.py part of nova's 8835866 was imported, but not the newly
added exception.

Change-Id: I0d79a8536f0c8f14cb062dc61c5fce100b63fd64
Fix eventlet/mysql db pooling code

Eventlet versions >= 0.10 return a tuple from
db_pool.ConnectionPool().create(), not a scalar.

Also, set the FOUND_ROWS client flag, which sqlalchemy
appears to require to sanity check the number of rows
touch.

bug 1097992

Change-Id: I64e798660c506fcd13947a232cb7832db0318360