ML2 plugin cannot raise NoResultFound exception

The ML2 plugin cannot raise NoResultFound exception because it does not
use the correct sqlalchemy library:
'from sqlalchemy import exc as ...' instead of 'from sqlalchemy.orm
import exc as ...'

Closes-Bug: #1277914
Change-Id: If3819adc62b9254f0c08eea6dcfcf5f06288e20e
Merge "ML2 plugin cannot raise NoResultFound exception"