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
(cherry picked from commit f18dbd7e74303b7e0af97439ed935038e2231feb)
Merge "Handle DB2 disconnect" into milestone-proposed