Need to use _() to handle i18n string messages

Partial-Bug: #1220502
Change-Id: If31dd659794743837e2e494a2b6ae4488ef6ca32
Fix mysql checkout handler AttributeError

If the mysql_on_checkout handler encountered an error due to
disconnect, it would fail with an AttributeError,

 AttributeError: 'module' object has no attribute 'warn'

This is because it used the logging module where the LOG logger
should have been used.

Change-Id: Iae8a9bd4aeee6e35508059c20b92d70c36232963
Closes-Bug: #1234873
Handle DB2 disconnect

If the DB2 server was restarted, database connections would fail,
requiring a restart of the Keystone server.

This change adds an on-checkout handler for DB2 similar to the
MySQL handler.

Change-Id: Ie53d9606edac408401961fc6526bb7c99baa836a
Closes-Bug: #1231657
Sync db.sqlalchemy

This brings in db.sqlalchemy from oslo-incubator commit
c2dcf6e81906288d3f3d66e2686635522ac69f71

Change-Id: I35eebedcc12500b6279634a845e5deca0614a642
Merge "Sync db.sqlalchemy"
Merge "Fix mysql checkout handler AttributeError"
Merge "Handle DB2 disconnect"
Don't use default value in LimitingReader

We can't simply pass the None default on to the read operation as this
default is handled differently between different wsgi implementations.

Change-Id: I337e797b8dee3dfcf9299fe361cf197a176c8fe2
Fixes: bug 1213106
Merge "Need to use _() to handle i18n string messages"
Merge "Don't use default value in LimitingReader"