Oslo sync to recover from db2 server disconnects

A checkout listener can be added to sqlalchemy to be able to
handle database disconnect pessimistically and avoid an error in
the event of a DB restart or simply a temp connection issue due
to network glitches. At the moment only mysql is enabled with
this feature. This patch enables db2 as well.

Oslo version in change I5563360b43c4b648f5b035099b0e1d2e71d32503

Partial-Bug: #1231657
Change-Id: I218ef9836f2e0f135897007d2e53354eb1248422
Merge "Oslo sync to recover from db2 server disconnects"