Move dnsdomain_* tests in test_db_api to own test case.

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

Missing test_dnsdomain_unregister added.

blueprint db-api-tests

Change-Id: I9c11ff723d3001499ada54786d09ff397e8fb6e2
Merge "Move dnsdomain_* tests in test_db_api to own test case."