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
Merge "Handle DB2 disconnect"