Conditionally import ibmdb2/ibm_db_sa

Since ibm_db_sa is not part of sqlalchemy, we need to handle the
conditional import of the module in visitor.py so we don't get an
ImportError if ibm_db_sa is not available.

Closes-Bug: #1287229

Change-Id: Ida070b629ce3b9be727ae49973bb6a71543c1dcf
Merge "Conditionally import ibmdb2/ibm_db_sa"