tempest/config.py parse some sections incorrectly

Two issues for tempest/config.py:
1. When parsing sections of object-storage and dashboard,
   wrong variable is used.
2. We'd better rename BotoConfig/ObjectStoreConfig
   to BotoGroup/ObjectStoreGroup

 Closes-Bug: #1231595

Change-Id: I0d3162708237211ce7beb4a651f11db57d4d5e44
Merge "tempest/config.py parse some sections incorrectly"