[FIX] sql_db: don't log dsn with unmasked passwords

The dsn may contain the connection password of the database when not accessed from a psycopg connection object.
Replace the unfiltered logs to use cxn.dsn avoiding password leakage in logs.
Fixes #1433
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64