Move database fixture to a separate test case

This change moves database fixture from ironic.tests.base.TestCase to
ironic.tests.db.base.DbTestCase and adds dbapi field there. Also it
adds ironic admin context to ironic.tests.base.TestCase, as context
may be used without db fixture.

Closes-bug: #1376285

Change-Id: I008e44da315767afb6282000a01f0b39698f9768
Merge "Move database fixture to a separate test case"