Add description for DB driver API
Add description for DB driver API

Adds to pluggable DB documentation

Change-Id: Ia982d29e237f0990ed131d19064fe315fb9bfc87

openstack/dragonflow,FrankDuan/df_code,openstack/dragonflow,FrankDuan/df_code,FrankDuan/df_code,openstack/dragonflow