Repair string-based disconnect filters for MySQL, DB2

The two regexp-based filters in exc_filters.py->_is_db_connection_error()
were both incorrectly formed.  The tests for these filters would pass
because the fixture also set the is_disconnect flag to True, which
normally would be set by SQLAlchemy, and therefore the
_raise_operational_errors_directly_filter() or possibly
the _raise_for_remaining_DBAPIError() filters would catch this,
view the is_disconnect flag as True, and promote to a DBConnectionError.
However, the _is_db_connection_error() filters are intended to
work independently of whether this flag is set; so two new tests
are added which unset the flag, and ensure that the exception messages
as given are caught here.

Change-Id: I37ddd669b89669730ae1ff07c7bc7a6ba5705f67
Merge "Repair string-based disconnect filters for MySQL, DB2"