Add index modifying methods

Add to openstack/common/db/sqlalchemy/utils.py
methods for modifying indexes:
- add_index
- drop_index
- change_index_columns
- index_exists
- column_exists

Add tests for these methods to TestUtils class
in tests/unit/db/sqlalchemy/test_utils.py

Change-Id: Ib7b2a1a8169fff803a925326703e71c014230a68
Merge "Add index modifying methods"