Identify some 'heavy' tests (that depend on ibm_db or mysql)
 - If ibm_db/mysql are not available in CrawlDBI.py, don't define those
   classes
 - Mark tests that depend on db2 or mysql as 'heavy=True'

Change-Id: I8e0cfa1645e95a77e1c8e93cd1112e8e27f4c7c9