Move bw_usage_* tests in test_db_api to own test case.

There is bw_usage_* tests in old mixed DbApiTestCase.
This patch moves them to its own test case BwUsageTestCase.

Missing test_bw_usage_get added.

blueprint db-api-tests

Change-Id: Ie79d31694d9c72d9d6f43772aeadff200987646c
Merge "Move bw_usage_* tests in test_db_api to own test case."