The ibmdb2 code calls _index_identifier() when it handles index name. This
method only exists from sqlalchemy 0.6.5 to 0.7.*. Nova code change
https://review.openstack.org/#/c/153123/ attempts to drop a db constraint and
it fails to sync nova db with sqlalchemy 0.9.8 running against db2. Need to let
ibmdb2 code identify sqlalchemy version and call the correct method to handle
index name.
Closes-Bug: 1428477
Change-Id: Ie6333f9cea0209c1ea290356873a1a1bcf409bed