Refactor the DB implementation of Capabilities API

Replace the update of DEFAULT_CAPABILITIES into the __init__ function in
the Connection class of base.py instead of the Connection class of each
DB driver.

The AVAILABLE_CAPABILITIES dict was divided into two case of MongoDB
and DB2 drivers, the common capabilities are listed in pymongo_base.py
in COMMON_AVAILABLE_CAPABILTIES, which is updated with the capabilities
dict of the two affected DB drivers using pymongo.

Change-Id: I244eabc64f985d5a7ab5e166c72bcdb88b7e3372