Don't raise MySQL 2013 'Lost connection' errors

cherry pick, including original commit message below from
oslo-incubator commit:

b4f72b2987d53bb434e3a833f06d70a2167f5efc

Closes bug: 1275838

Although this code error is part of the `ping_listener` tests, it was
missing from the list of known - and ignored - connection errors.

The code error 2013 refers to a connection lost during a query, see:
https://dev.mysql.com/doc/refman/5.0/en/error-messages-client.html#error_cr_server_lost

Without this error code, the session won't try to reconnect to mysql,
which makes `max_retries` useless.

(cherry picked from commit 472c1440677440e0888570926b49542ddf260724)

Conflicts:
	heat/openstack/common/db/sqlalchemy/session.py

Change-Id: Id8a1c0665468b503b092c4f01ffd40e2be9c6fa9
Merge "Don't raise MySQL 2013 'Lost connection' errors" into stable/havana